I came up during that middle era when the shift was happening. I was an early adopter of Linux, but all of the real training I got (my employer at the time paid for it) was Solaris-based. But, even with the training and access to Solaris, I prefered Linux. I just had more comfort with it because it was my daily driver. When people I worked for were making decisions about OS, the recommendation they nearly always got from me was "Linux". And, I believe that played out millions of times to get us to the world we're in today.
So, yeah, Solaris was competing with free, but not always at the business level...the part that mattered was "how many people with influence are using this OS as their daily driver?" And, Linux was/is a phenomenon. People love Linux. People loved Solaris, too, but it was a much lower number due to lack of access...early days of Linux, you couldn't even get Solaris without a SPARC box to run it on. Later, they made x86 Solaris free, but it was too little too late, and by that time Linux was better than Solaris on a number of extremely important metrics (package management and package selection, for example, but also in terms of just plain fun).
At work GNU/Linux was just an internal server, all real work was being done in Solaris, HP-UX and Aix servers.
So fast forward to modern times and even Microsoft uses Linux kernel syscalls on their new POSIX personality subsystem, instead of actually supporting POSIX.
POSIX support is in the API, not the syscalls. musl libc places great emphasis on POSIX conformance. So if you ran a musl-based distro atop WSL, that should give you what you want, or at least something closer.
Is that true? Don't companies interested in Solaris pay for their OS? Weren't they competing against companies like Red Hat?