> Why would someone who is developing their software on an x86 box with Linux care about ... Solaris?
There was a time when ZFS would have been a good answer for this. Of course, now it's available on FreeBSD and Linux.
At one point also there was zones, but again, containerization is pretty standard elsewhere now.
I guess also if you go further back, like the Linux 2.6 era, which is close to when Solaris was open sourced ... the 2.6 release involved a concerted effort to improve SMP support. So maybe you'd be interested more in commercial Unix in the 2.4 era, for better SMP. But again, Linux surpassed this ~20 years ago.
And not overcommitting memory.
But Illumos is a real modern viable option over Linux (and *racle Solaris) for server hardware.
On the onboarding thing, Oracle Cloud didn't offer Solaris as an OS option last time I checked. And, of course, they never offered SPARC because abandoning it was one of the first moves. IIRC, IBM Cloud also doesn't offer Linux on POWER (but they do offer AIX and IBMi, as well as z/OS - and Linux - on s390x). Not that long ago I tried to convince VMSI to make a deal with cloud providers and allow OpenVMS to be as easy as any other OS on amd64.