But if a carrier has the nerve to actually block updates to my phone and I find out, I'll switch my phone to another carrier the next day.

What's in it for them?

Most people don't think about such things when buying (renting) a new phone. If you do consider it, you're an exception to the rule.

What do you mean rent?

I buy my phone. It's mine. I can put it on any network I like.

You certainly can't put whatever software on it that you like. They go well out of their way to ensure that.