The copyleft nature was essential to good driver support. It set it up such that for corporations making drivers the easiest path was to get the driver upstreamed. There was a bunch of hoops they could have gone through to avoid that (as many did, like Nvidia) but that became a sorta-default.

Copyleft encourages a collaborative relationship between entities because it makes trying to play it close to the chest with IP involve more legal effort (if it's possible at all).

Yes, I can see that stalling development as (at best) it turns into a pile of private forks rather than a cohesive project, but from what I remember that was already after Linux had "won" the "Open Source Kernel" race.

Commercial support for Linux was... Sparse... before the early 2000s.