I think that Sinatra will become increasingly popular within the Ruby community (and financial help from Heroku/Salesforce can't hurt!), but I also think it's interesting that the micro-frameworks inspired by Sinatra (Flask for Python, Express for Node, plus many others), will become the fall-back for more advanced developers because they do a lot less specification and leave a lot more space to the developer.

So I say don't give up on Ruby. And if you insist on Rails-style MVC, there's always Padrino for Sinatra, Tower for Node, etc.

So much going on, so little time...