Personally, I think the ruby world really lost something when merb folded into Rails. It left the community split between the one giant framework and many microframeworks, where merb was really something in between. Big enough for real work, but small enough and with a small enough dose of 'magic' (where rails has an absurd amount) that it was still pretty understandable.
Rails became more configurable because of it, but I don't really feel like it ever got smaller or more understandable. That said, I've been out of that game for quite a while, so maybe it eventually did.
But I also don't think it's really comparable because merb wasn't a fork of rails like io.js was of node.js.
Padrino might be a good counter-example, because I think maybe it occupies a similar niche as merb fell into (I can't say for sure, because I've only used it very experimentally). But it took a long time from the merb-Rails merge to Padrino existing (at least 2 years, I believe).
Rails became more configurable because of it, but I don't really feel like it ever got smaller or more understandable. That said, I've been out of that game for quite a while, so maybe it eventually did.
But I also don't think it's really comparable because merb wasn't a fork of rails like io.js was of node.js.