You're never going to love every decision made by the designers of any platform. The thing is, you're likely to make just as many missteps on your own. Every once in a while it's worth it to start with a clean slate but I don't think Rails is even close to that point yet.
I don't think Merb will replace Rails. I think, ultimately, the two appeal to different crowds. Too some extent, I hope people who are happy with Rails stay with Rails. Merb feels like it's more aligned with the Ruby community than the Rails community. If that makes any sense.