The only thing I've seen last "Repeatably" beyond 3 years with stock internals is tokyo marui.
You can have the odd G&P or CA, or even an ARES last 4 years bone stock. But is EVERY G&P, CA and ARES going to last that long? Definitely not. But on average, marui is hands down the single most reliable AEG on the market.
For everyone else, there's upgrade parts lol
Keep in mind, the upgrade/replacement parts are only as good as the gundoc installing them. A shitty gundoc can ruin ANY gun.