Takeaways:
1. How is Ruby crossing into the mainstream? It's been mainstream for quite some time. The fact that it's not the number one language does not mean that it's still some little niche language. Moreover, what does it matter if Ruby has crossed some arbitrary threshold into "mainstream"?
2. Ruby is not a "better platform". It's a different platform. Ruby and .Net each excel at some things. Some of those things overlap and some do not.
3. If you're not excited about .Net, then don't use it (unless you're being paid to). Rambling about how it's stagnating and playing catch-up (aren't those contradicting?) is pointless and shows a lack of historical understanding. How much effort was poured into Ruby to build the libraries and frameworks that were already available in Python, Perl, or (god help us) PHP?
4. People identify themselves as X-developers all the time. It's not a negative thing. Ruby developer, Java developer, .Net developer, C developer. Whatever. Single out ".Net developer" does nothing except demonstrate personal bias.