I don't think someone being stuck with Solaris for unknown reasons is sufficient to declare that Solaris is better or equal to Linux. For the ATC, it could simply be a matter of "The application we run only support Solaris"
It's aviation. Even if there was a superior solution and it was readily available, it would take decades to get through certifications and before it became widely accepted or even the new default. See GPS vs radio navigation or just analog radio communications.
Yeah that would be on top of it all. For the ATC to use Linux on something, there would have to be a competing application that runs on Linux, which has to be certified and then migrated towards. That could take a decade or two.
See also this whole radio-altimeters vs 5G debacle that wouldn't have happened if radio altimeters used a reasonably modern design.