There's a lot of enterprise server-side software that, for better or worse (I'd argue worse), was only developed for Windows Server and will probably always only be developed for Windows Server. So at the very least you'll need some Windows VMs to run these (or maybe Wine, but it's highly unlikely to receive any sort of enterprise support that way, which is kinda the point of using enterprise software).
The good news is that a lot of it is written in CLR-based languages (particularly VB.NET and/or C#), so porting to cross-platform versions of .NET (e.g. .NET Core, .NET 5+, Mono, etc.) is at least theoretically feasible if developers can be assed to do the porting. Barring a solid business case for it, however, it's unlikely.
The good news is that a lot of it is written in CLR-based languages (particularly VB.NET and/or C#), so porting to cross-platform versions of .NET (e.g. .NET Core, .NET 5+, Mono, etc.) is at least theoretically feasible if developers can be assed to do the porting. Barring a solid business case for it, however, it's unlikely.