Mono is perceived as a second-class citizen in the Unix-like space. Java, its main competitor, integrates very seamlessly and has excellent runtime tooling support Mono doesn't rival. Mono may gain some traction in some niches, but the Unix-like ecosystem is exceedingly diverse, without any dominant tool. Mono could double its presence and still be barely detectable. It could displace some of Java, but that would not affect the majority of the space.

It's impossible (and foolish) to avoid C# if you develop for Windows, but I haven't written a single line of C# since 2004.