No, a good number of Rails devs stayed around and it became more of a mature community, but the hype driven growth/mindshare was over. There is a certain loud percentage of webdevs driven by fashion, and that portion follow the latest fashion around around. Rails lost some to Node, Node lost some to things like Go and/or React as API backed SPAs grew.

I really doubt many Rails devs went to .NET. In those days Ruby was pretty bad on Windows machines (might still be?), and nearly all Rails devs were Mac users.

I never knew .NET MVC was Rails inspired - I always assumed it would've been closer in style to the older Java MVC frameworks Rails was keen to disrupt in the beginning.