It's not just the current gap, but the absence of long-term guarantees that the ecosystem will stay on the current open-source trajectory. Microsoft can close SQLServer back down tomorrow; nobody can take away Postgres (even when someone tries, like Oracle with MySQL, the community reacts pretty quickly).

C# is a nice language and if you stick to the classic MS ecosystem you can be very productive, but if you're not invested in Windows and you're not ready to spend real money for it (either right away or at short notice), you should probably look elsewhere.