iPhone upgrades - a one-way control-freak street

It's a tradeoff. You either support a bunch of versions on a wide array of different hardware, very badly (c.f. Microsoft -- we diss them, but to be kind, the problem they have set up for themselves is truly challenging), or you try to do a better job of support, in which case you must constrain yourself to supporting fewer options.

"After all, it's my iPhone, isn't it? Or is it?"

The phone hardware is yours once your subsidized contract is up with your carrier or you pay the early termination fee. The software is licensed. I don't know of any modern SmartPhone platform that makes juggling operating system versions easy.

I didn't upgrade my iPhone from 3.0.x to 3.1.x for months because I wanted to keep tethering through the unofficial carrier hole as my carrier then didn't support tethering. I ended up paying for the SIM unlock and porting my number away from that carrier.