It does tho. Not all cassettes are built on the same tolerances and different brands might have different interpretations of the standards, so always check the limits.
Cable tension more often then not can be dialed on the derailleur as well.
Worn chains can impact shift performance, and sometimes adjusting tension or derailleur height can improve things.
These are things that aren’t noticed when everything is new and clean, but can slowly creep up as an issue.

So yes. I agree with your sentiment that a well set up derailleur should keep shifting, in practice it’s more nuanced.

In NL/BE it’s so bad that most settled bike shops won’t even plan your repair if the bike was not bought at that store. Why? Because there’s still more then enough work with only your own customers.

So you’d ask yourself, why don’t repair only places open? Because the knowledge required to start fixing bikes has evolved a lot with the multitude of e-bike systems, and moreover people already find the repair prices too expensive when the costs are slightly subsidized by the people buying bikes at the shop.

Well maybe (even though I do think a bit of a bent hanger can be compensated for) but even then it’s only true until you change cassette/chain or the shifter cable, which you will have to do at some point.

Well if visiting a (compromised) site can get you infected the problem is not the site, but your browser, right?