There are a few Marui original AEGs out there running bone stock on at least 50k+ shots, I know people who've maybe swapped out a piston and gone way past 100k on the same sorts of guns. Yeah it varies between brands, but over-maintenance is very much a thing and taking stuff apart causes wear, wear that's rarely needed to be caused. AEGs are not in general particularly dependant on cleaning and re-lubricating in terms of the inners of the gearbox, I've got multiple electric guns in my cupboard from different manufacturers that've sat there for nearly 10 years without ever being cracked open and still work exactly as expected. Maybe gain a little range with a new hop bucking? Quite possibly, but not much.
If you did somehow get sand or grit in to the grease on your piston rails or whatever then yeah that's definitely a serious problem, but congrats if you do manage that. Most AEGs are a lot like RS M4s, extremely closed up/closed system. Once you've got a mag in, the chances of crap getting physically inside the gearbox are seriously small, can't say I've ever seen it myself. I'm sure it happens occasionally and if you dive on a pile of sand with no mag in your gun and badly-mated set of receivers then maybe.. but even then it's unlikely.