If Microsoft were to abandon .Net then I'm pretty sure the motives for maintaining Mono would slip as well.
However I think that is extremely unlikely, no doubt it will be evolved in that annoying Microsoft way but completely abandoning it would make no sense at all.
They won't abandon it. Most of their server software is written in .NET: ASP.NET, Exchange server, etc. Most of their administration tools have been rewritten and now all use Powershell (which also has become the primary interface for server management by admins) under the hood.
However I think that is extremely unlikely, no doubt it will be evolved in that annoying Microsoft way but completely abandoning it would make no sense at all.