They wouldn't really, unless they are that far gone they can't supply a decent stable current and keep a stable voltage any more.
Mine are all 3 years old or more, running a 5.5 in 4WD and there's no-one else at my club can get away from me down the straight. Just the bits where I crash
It's only the rich kids and those who are fighting for F grades that replace them every week/month/year. I'll run mine until the car doesn't move or they are swollen. Sure, I look after them by putting them in storage and checking the IR of them, but any that are higher than the others just get used for practice and heats - save my best ones for the finals.
Agree with everyone else, a 20T pinion is way undergeared. I use a 23 in my 2WD as does most the others at my club - and that's outdoor astro. Indoor carpet you can ramp it up a bit. You'll hardly notice any loss of bottom end as modern motors are just that good. Just turn the punch up a bit if you do.
It deffo sounds like the ESC settings need double checking though pal.