Some people want the wider ecosystem, and for their phones to be treated essentially like desktop computers - the wild west, with endless possibilities. And that's fine. That's not what Apple is selling. Apple is selling a controlled environment - a massively capable device that is decidedly NOT a desktop wanna-be. The fact that the hardware could do it isn't actually relevant. Ferrari isn't selling dump trucks, even if their cars have four wheels and a trunk.
For the vast - VAST - majority of users, this means sweet bugger all. No impact. No benefit. Some minor contingent of nerds will care... but they mostly went android anyway. But this isn't about the users. Not really. There's no new capability being introduced here.
Now, for developers... that's more complicated. Will anybody experience the intended benefits? Maybe? I'm skeptical. I'll bet some that try will discover that the costs to implement this side-loading from their own infrastructure outweigh the sales. But that's just a swag.
But the second somebody comes calling for warranty support because a third-party app is misbehaving, I would expect the default to be for Apple to say "no", and to either start the per-hour charging meter running, or to insist on a factory reset before servicing it.