I think you're missing an important point here: the right to fork is the primary point of Open Source. Sure, it's nice if you don't have to fork, but ultimately the reason Open Source is important is so that you can fork if you have to. The other parts, about projects being able to accept contributions from others and so on, are nice to haves, but not essential. The right to fork is what is essential, so that when a project has a bug, and the project owners aren't fixing it, which is expected to be the most common case (see below for why), you, as an individual, can at least fix the bug for yourself, instead of being stuck with a proprietary system with a bug you can't fix at all.

Why is it expected to be the most common case that the project owners won't fix the bug? Because the tradeoff between benefit and cost from the project's point of view is very different than it is from yours. For you, it's much easier, as you say, for the upstream project to accept your PR than for you to have to maintain your own fork. But from the project's point of view, it's much harder to accept your PR than to just ignore it. Sure, you might be convinced that you've fixed an important bug and your code will be a net gain to the project. But how do they know that? To them, you're just some random person on the Internet. It's not like they have people sitting around just waiting for random people on the Internet to make contributions. Any time spent on evaluating your code is time not spent on something the project has already committed to doing. And if they aren't feeling any pain from the bug themselves (which they won't be if you, a random person on the Internet, have had the time to see it and code a fix for yourself), fixing it won't be on the list of things they are already committed to doing.