If open source only works when you pay for it, it's just an abstraction layer around ordinary commercial software, with the same pressures. In fact, most of the people qualified to fix Linux video drivers are already employed full-time at a graphics card manufacturer (with its sales pressures) or a commercial redistributor (with its sales pressures). You could pay one of those companies for a support contract. But other than that, if someone else qualified exists, it's only worth their time to take these contracts if there's enough sustainable business to keep up-to-date with how Linux graphics drivers work; chances are there isn't, and they're spending their time in full-time employment doing something else entirely. So, you're at the same place where you started: the problem is complicated enough that there aren't enough reliable sales, and so it only gets done by large companies that can afford to fund the work because they already have enough sales of enough things.

(Full disclosure: I am one of the people who could have been qualified; one of the jobs I was considering a few years back was a graphics-driver-hacker position at Red Hat. I happened to instead choose full-time employment doing various, mostly userspace Linux stuff for a startup. When they ended their incredible journey, I would have loved to make a living taking contracts like the ones you suggest - and I still wish I could - but there aren't enough of them and they aren't reliable enough to make it better than just taking a full-time job with a profitable company.)

With ordinary proprietary software, there's only one team in the world you could possibly hire for maintenance, and if they're too busy or have a conflict of interest you're just screwed. Competition among maintainers should make them more efficient and scale better.

Maybe?

But that assumes there is a ready supply of people that can fix a certain part, and want to take contracting work. For example, let's say I have an ATI XYZ in my laptop. It has a driver bug and won't work. ATI driver is open source. Who do I hire to fix it?

Except that's not quite true for ordinary proprietary software - there are a number of people who make a living supporting Windows or Active Directory or Exchange or whatever who don't work for Microsoft. There is a difference of degree, in that third parties who support open-source code (e.g. Oracle supporting RHEL) have access to the source, and third parties who support proprietary code usually don't (although it's possible to get a MS read-only source license!) and only have access to compiled binaries. But that's not a huge difference. There are people who are very good at tracing and disassembling compiled binaries, or simply at understanding how the system works even in the absence of code, and they have thriving businesses. Sysinternals, for instance, was a separate business acquired by Microsoft.

(Note that I am carefully using the phrase "open source," not "free software". Part of the free software ethos is that you can realistically modify your code as needed. I do happen to think that the current free software movement isn't very good at delivering on this promise.)