By the time the 360 came around, those tools and APIs had matured to the point where it was pretty trivial to release on PC if you were already developing on Xbox. That wasn't true of the PS3.
Measured by the PS3, porting was easy, but still far from trivial.
Countless butchered ports during the 6th gen, mostly caused by the limitation of Microsoft's DirectX 8 API (try to get soft shadows to work efficiently).
The 7th gen didn't fare much better, although d3d9 and especially d3d10 were far more competitive to Xbox's low abstraction version of DirectX.
The merge happened in 6th gen.
You mean transition.
As PC was held hostage by the WOW craze and Steam wasn't as massive as today, PC gaming was often called dead since the mid 2000s.
It was often oversold and depending on the genre and ip the PC market was still profitable but not enough for keeping exclusives.
I wouldn't be surprised at all if, in this alternate universe, studios like Bethesda and Infinity Ward just ditched the PC completely by 2008.
And how did Microsoft keep those games on pc? And Bethesda, really?
Microsoft didn't even port its own games to pc.
Remedy had to port Alan Wake themselves, Mass Effect got ported by EA, Halo 2 came far too late and got cucked by an artificial Vista dongle.
And as already mentioned, Xbox DirectX and PC DirectX were never comparable.
Also, Microsoft and Sony switching to x86 wasn't because writing C/C++ code for ppc is any trickier than for x86
I never ascribed any intend.
It was just one of many factors I listed why the merge happened with the 8th gen.