Working against Rails, or any framework, is definitely going to cause pain. On the other hand, Rails is not perfect and I, like the OP, have been noticing a number of voices lately worrying about building large Rails apps. Which is awesome, because either Rails will adapt or something new will come along to fill the need.

Sinatra + cherry picking rails lib might work great if you've built a few apps like that, but it would nice to package this approach in a framework which guides people down this road.

Personally, I feel that Rails could evolve to the point where the approved path involves using some adapter (like a service layer) to talk to ActiveRecord. I've written about how Java accomplishes this in comparison to ActiveRecord: http://casestatement.tumblr.com/post/11514731433/javas-jpa-f...