"Less popularity means less jobs available, less career opportunities, less projects to handle and so on."
What you're not mentioning here is that "less" isn't necessarily a problem. "Less" than a helluva lot is still a lot. I've been working as a C# .NET developer for a whopping 7 months, and my LinkedIn inbox gets about 3 or 4 unsolicited messages per month from people looking to hire. This has been going since my second week on the job.
Might there be less C# jobs than JAVA jobs in the market? Sure. Will you struggle to find work as a C# developer? I seriously doubt it.
Sure, but in a small city, less could means there are only 3 jobs compared with 30 in Java, and that could be a struggle.
Less in a small industry could be the same.
Anyway, learning a strong type language like C# is a strong addition in any developer arsenal.
I think different stacks have different popularity by region. Here in Stockholm (and Sweden in general) nearly every job listing is for either a JS full stack type position or C#/.NET. Java is way less popular here. So OP should definitely check their local listings first.
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