Branding and recognition. Microsoft wants to be able to say C# is the top 5 most used programming language. Not that they have 10 different languages in the top 100 used ones. C# is the flag ship and there shouldn't be any question whatsoever which language you should use if you run the MS stack (unless you have very strong desires for something special, like F#). Splitting the language into a new one would just segregate the community and you risk ending up with the python 2vs3 mess.
Also consider working in enterprise and asking your manager if it's ok to "change language" from C# to MSNEW#, vs doing a "minor update" from c# 6.0 to 7.0.
Even if MSNEW# was backwards compatible and C# 7.0 wasn't, i imagine the answer on the second question to much more likely get an ok.
This has no been the case historically, I'm not sure why it would be now. Why would ms invest in languages like vb.net and f# if so? Choice of languages that target the CLR has always been a selling point even though c# is dominant.
Also consider working in enterprise and asking your manager if it's ok to "change language" from C# to MSNEW#, vs doing a "minor update" from c# 6.0 to 7.0. Even if MSNEW# was backwards compatible and C# 7.0 wasn't, i imagine the answer on the second question to much more likely get an ok.