Although I went through a phase of thinking of upgrades even before buying a bike, now I'm more on your side, touching the bikes the bare minimum to keep them working. If they are original, I also get the feeling that if the bike has been kept like that for 25 years I should not change that, as long as it's working.
My kids are currently on 16" and 20" bikes, that already use "adult" components. The temptation to upgrade was huge, as most stuff is really basic, but since the bikes work and they will outgrow them in 18 months, what is the point? I can resell them cheap (bought them used anyway) and finance the next size without much investment.
Over upgrading also has the negative effect of making it harder to sell your bikes, because if you invested so much money you don't want to lose it, but on the other hand you'll never get your money back. And I always laugh at how we lie to ourselves when we say "this bike was nearly free because I used stuff from my parts bin", as if the bin got full on its own, magically and free.
The only moment when I allow myself to upgrade is when something breaks or wears out, as I feel I have "earned" it. Nothing crazy, I have no garage queens but limited time and budget!
Finally, the sign for me that Shimano is superior to the other drivetrain manufacturers is that nobody else can make a derailleur for 15£ that you can find still working in 10 years. It's easier for SRAM or Campagnolo to make good stuff when you charge £££ for it...