For a while, I thought Sinatra was taking that role? It might be interesting to think about what people are looking for that's neither Sinatra nor Rails, or why Sinatra didn't live up to what people were hoping (if people think that).
Sometimes I think some of this is just utopian grass-is-greener thinking. While there are _many_ things I'd do differently in Rails if I had the choice (some but certainly not all of which the Rails core team probably agrees with, if they had the chance to start over)...
...I think some of the "the thing we need is something _lighter weight_ than Rails" thinking is basically wishful thinking. When you start with something lighter weight, you (being me) generally find you need more than it offers, and then you've got to go finding your own things to do those things, and when these extra third party things end up not as high quality as you'd like, or end up abandoned by their developers, or you end up spending many hours re-inventing a wheel you're sure someone else has already invented.... either the 'lighter thing' ends up gaining weight, or you end up wishing it had.
Doesn't mean I agree with all of Rails choices about what to include, or how to architect it. But I think people under-estimate how challenging it is to hit the sweet spots, as if Rails core team just lacked the will or intelligence or proper understanding or something, none of which I think they lack. Still, certainly alternatives are great, testing grounds for other possible ways of doing things are great, the more different things we see, the better all of our architecting and coding gets, that's the only way to learn.