I had a recent, lengthy, conversation with a recruiter. He said two things:
1) Microsoft dev stack is really a dead-end, any new startups or even large corporations starting new initiatives are more and more are moving away from any kind of MSF tech or Azure cloud development, and
2) no developer really wants to work with MSF tech.
I have lived in the Microsoft world my entire career, although MSF is increasingly open source/polyglot. Also, I spent a couple of years doing node.js, react, salesforce dev and integrations, Heroku, etc. I found it a breath for fresh air in some ways but lacking in others. And I have to say that I do 'like' working with C# and Azure and the rest, but I am at the point in my career, and with the market the way it is, that I could still possibly make a move and not take a major hit salary-wise, but I probably not for too longer.
For context I am approaching 40 and live in a major US city in the southeast, and I do not have an interest in going into management.
Secondly, some recruiters are liars who will tell you literally anything to try to get you to apply for jobs they're recruiting for. A recruiter saying MSFT stack is a dead-end just means they don't have many MSFT stack jobs on their books. If they had one they'd put you forward for it, and not tell you that they think the stack is a dead-end.
MSFT's stack is not going to disappear any time soon. It's growing just like all the rest of the industry. There's definitely no harm in learning other things, but don't write it off if you want to carry on with it either.