I have an 11 mile route to work. 4 miles of it is normally gravel (right now it's more like 6 due to construction detours), which means no chance the bike stays clean. A clean chain is covered with sandy clay within 2 miles. Washboards and potholes are pretty much constant on the gravel.

I think it's possible that part of the reason my derailler stays in proper adjustment is that I have a kickstand, and my bike has NEVER been laid down or fallen down as far as I know. That's got to be hard on the derailler, and is one reason I won't have a bike without a kickstand.

I'm not a very speedy rider, I'm only averaging about 17-18 MPH over the 11 miles. I do have some hills and until I built a new rear wheel the load/unload stress back there broke about 20 spokes, so there's some stress there.

In the winter I take the same route but with studded tires, that gravel road is pretty much solid ice all winter.

I have replaced the freewheel twice now, because I screwed up and let the chain stay on way past its worn out stage, and when I replaced the chain it started skipping like crazy. But even then I didn't have to touch the shifter.

The only times my bike hasn't shifted right, it didn't need a derailler adjustment, it was because the chain was so gunked up it would barely bend anymore. OK, also one time I had my tire pump twist around on me and pull sideways on the open part of the shifter cable, and I thought it needed adjustment until I found that, twisted it back, and everything was back to normal again.

FWIW, I'm really easy on equipment in general. I didn't replace the brakes on my car until 100,000 miles, and even then I estimated they probably had another 20,000 in them, but I'd bought pads already and figured I might as well install them and not have to worry about them for another 10 years or so. That car has 130000 miles on it (bought new) and I've spent < $2000 on it including 2 new sets of tires and all consumables apart from gasoline. I tend to coast into stoplights slow so I can hit the green, I slow down early, etc. I do the same with my bike.