It really depends on the problem space, though. And for most reasonable applications, Rails/Sinatra scale just fine through well-understood horizontal methods.

It really depends on the problem space, though. And for most reasonable applications, Rails/Sinatra scale just fine through well-understood horizontal methods. By the time you're getting to where they don't scale well, you're probably longing desperately for a rewrite anyway, because your core problem has shifted from "Does this solve an actual business case?" to "How do I make this support a heavy load?"