I think where we disagree is that I don't think of the lower level API as being much of a lock in. The better graphic programmers I know have pretty extensive experience of the various flavors of DirectX and OpenGL. The general principles are the same and good programmers move between them easily.
Lock-in here doesn't mean they have no technical means of implementing other graphics backends, it means that implementation is hard.
A lot of common middleware supports Linux just fine. It's graphics that's usually the biggest hurdle. People have expertise to address it, but it's still a tax to pay. And different distros support is a very minor thing in comparison.
If graphics is not the biggest issue, what is then in your opinion?
Graphics is the biggest issue, but the issue isn't at the API level. It's in the driver and hardware differences below that layer.
The "tax" as you call it, comes mostly from the hardware drivers leaking through the abstraction. Part of this is AAA game developers fault since they are attempting to use all the GPU with edge-case tricks to eke out more performance.