So Linux, “the free alternative to Windows”, needs corporate backing to sell it and make it mainstream?

It does help. Just look what has been achieved thanks to Valve.

And I think the worst thing, the thing holding Linux back the most, is the divisiveness of the Linux community

True, but the linux community is SO much friendlier and welcoming now than it was 20+ years ago. And its getting better all the time as that kind of behavior gets further and further ostracized, alienated, and shut down.

I still remember the amount of toxic hostility and hatred I received from Linux communities in the late 90s and early aughts when i gave my first couple tries to linux.