MILSIM, as a term, has become a marketing term for Airsoft events that is now unavoidable. You have to use it otherwise your game is seen as inferior in some respects - the term MILSIM has less to do with creating a military simulation and more to do with being a subtle suggestion that newbies should stay home. It's a simple way of identifying that a game is not meant for people to come out for 8 hours and then leave.
I do not consider the vast majority of games labeled as "MILSIM" to be considered a simulation in any sense of the word. Nightfall is not a MILSIM. Almost every game I have hosted and labeled a MILSIM is not a true MILSIM. Games that have come and gone in years past I would still not label a MILSIM.
The only true Military Simulation I have ever participated in, atleast in my opinion, has been Operation Deadfall. Deadfall typically only attracts the hardcore players, is hosted at a time of year where the weather is usually unforgiving, and the terrain is usually just Canadian woodland. It attracts only the people who want to sit in a hole in the rain for 24 hours. It attracts the people who want to enjoy the suck.
To get to the root of this discussion you have to look at many of the ways things intersect and create the situation we find ourselves in.
First the 18 hour event we have seen the last year and a half or so is simply a business decision as it relates to PRZ. To rent the field for a full 24 hours, to run a game from 2pm on Saturday to 2pm on Sunday, means that the field is taken up for two full days. The field admission fee has to be priced accordingly. When you run a game from 2pm to 8am however, the entire field can be cleared out before Noon. This allows Paintballers to come on the field for Sunday. The price of admission for an Airsoft game is cut in half at the expense of a few hours.
The reason why we find cutting those few hours off beneficial is cause no one used them anyways. You lose 6 hours of game time but for most events by that time the field would be empty. Looking at past games, such as Rhino 1, the game was pretty much dead by that point. There were people on the field in the morning but hundreds had already left. While I woke up in the morning to a couple of gun barrels in my face because the enemy team had infiltrated into our building (and I was sleeping on the field), their infiltration was only possible because our team had no one in the building defending it. It was vacant, aside from me snoring in a random room.
What I have seen happen with the 18 hour events is that people are less inclined to wake up for a morning rush. If everyone fights until 2am in the morning, people are content to retire to their car and sleep. Once they've reclined their seats they don't want to get back up in the morning, even if the team leader is going window to window trying to wake people up. It seems to have created a problem where people just don't want to wake up for a final push right at Sunrise, even though that would be the opportune time to jump back into the game.
PRZ may also suffer from the issue of having the cars too accessible. Any game where the parking lot is easily accessible it creates the desire to just go back to the car. At PRZ usually atleast one of the teams is close enough to the parking lot to just go back to their car for a minute or two. This may also be a factor in people going back to their car. If it's too easy to do so - if the option is there, some may take it. At some of the other 24 hour games played deep in a forest it was harder to retire to your vehicle in the middle of the night, thus people tended to sleep in the field.
Even with all these factors a host can consider and try and plan around - it is still up to the players to stick it out. In this thread people have mentioned that non stop objectives all night help players stick it out, and this may be true, but it's not the only factor. Operation Hot Box, an 18 hour game hosted at PRZ (thus potentially prone to all the problems I have listed above) had an almost 0% attrition rate. Everyone stayed in the game, in the field, slept in the field, and made it until Sunrise. The objectives for that game were pretty simple and did not change for the duration of the event. Yet everyone made it to the end.
I have been to a number of events where the objectives were nonexistent and everyone had a blast. Even at OP NO NODS when the objectives stopped rolling in people still were having a blast throughout the night, but at OP:OP2 I had almost universal feedback that the constant, persistent objectives changing throughout the day made the game exciting and dynamic.
But when I look back at every game I have played at Finches I have usually had a better experience overall. I think this is because those fields aren't sexy. Finches is literally a forest. It's amazing, with varied terrain that change not only across the field but even over the course of the year... But it's not a massive 3 building complex with both interior and exterior terrain to navigate and fight over.
Most players who get into Airsoft do so because they think it's cool. They think the guns are cool, the gear is cool, and they want to play pretend soldier. It's why I got into it. Nothing is more pretend soldier than fighting down hallways and clearing rooms. This is what everyone wants to do. Less people want to go play in the bush. PRZ casts a wide net where everyone wants to play there because of how cool it is even if they aren't physically or mentally prepared to go the distance, but the outdoor venues in the middle of nowhere seem to only attract the people who really, really want to sleep on the field and fight with their friends for a full 24 hours.
Hosts play a huge factor in getting people motivated, but they aren't the other ones who can determine how the event will turn out. A host can make or break a game... But if the players just aren't willing to put in the effort nothing the hosts can do can fix this. Think of how many games where radios, kill rags and glowsticks are all part of the mandatory kit, and how many people just don't show up with them? I have about 7 yards of red fabric I bring to events now that I can cut up and hand out to players without a kill rag (or if someone loses it), but I can't exactly fix them not having a radio. What if they have a radio, but just don't want to work with their team and instead be a lone wolf?
There are a huge, huge number of factors that go into these events and why some make it and some don't. There is no one simple answer. If we are talking about PRZ exclusively, I'd say the sex appeal of the field and the 18 hour events kill it. The field is too irresistible to turn down for a number of people, whether or not they had any intention of going the full 18 hours. When a player realizes that they can keep going until 8 am and have to sleep all Sunday, or go to sleep at 3 am, wake up, drive home, and be in your shower by 2 in the afternoon and still have a Sunday to do whatever with? I've seen a number of people take the second choice - and I have myself on at least two occasions I can remember.
The issue with this discussion is we won't get an answer out of it. There is never the ideal format because people like different stuff. I think the first step in the right direction would be to not call these games MILSIM's and reserve the term for the most hardcore and dedicated events, but that isn't going to happen. Once you stop calling it a MILSIM the players that are attracted to those games that want to go the distance won't come because "it is not a MILSIM." Then all you are left with are the people who won't go the distance, and you end up with a worse problem than the one you started with.