The thing is seL4 had (has?) many limitations which made it not very interesting for the real world: at some point it wasn't able to use multiple cores, or use low power states.. I think that only people who have closely looked at seL4 can tell whether it's really usable for their use case.

Linux started out as a hobby project.

Sure, and I used it back then. There were kind of no other realistic options with momentum behind them. But also we just accepted that it was a bit cowboy and that was part of the fun. I guess it could have been FreeBSD or even Minix that got the momentum maybe, but Linux caught on with enthusiasts, it made the right compromises and then big names like IBM put weight behind it.

I think that the 'big names' that threw their weight behind it mostly did so because they realized that (1) they'd never manage to get a community like that going and (2) because it did not seem to compete with their core business.

SUN, DEC, SGI and many others besides that aimed to dominate the server market were left by the wayside, and even Microsoft was running (very) scared. If not for their illegal behavior they too would have lost the server market.

Yeah it was a crazy time to watch the whole bottom of the workstation market fall out. Solaris managed to hold on in datacentres for a while, but the others not as much. And a 486 or Pentium running Linux was just as good as most proprietary Unix at the time, or was well on its way to getting there.

I did a summer contract job at an IBM subsidiary in summer 98 or so, and I had a manager there fight me on installing Linux on some spare PCs, because he thought they should be running, y'know, SCO. A Real Unix. It already looked preposterous by then.

One of those SUN pizza boxes held out so long that the bearings on the drives were completely worn out, everything was moved onto a different machine and when it was powered down you could hear the harddrives have a most horrible crash. The drives would not even spin when powered up again.