But that doesn't diminish any of my points. So he did reimplement ActiveRecord. I'm not surprised he found it difficult to actually integrate into Rails. Rails is a framework. You don't treat a framework like a library. It's not going to play nice with your design because frameworks by their very nature impose the design on you.
With a framework, "works for me" is a perfectly valid reason to keep using it and to dismiss criticisms with. That's the whole point, to get you up and running with as little fuss as possible.
I personally hate CSS frameworks, I'd rather retain more control over the markup and selector semantics. That's a choice I make, and I'm aware of the tradeoffs. I would not make the same choice with web dev, because choosing the other side of that tradeoff involves way too much work.
But I'm a working coder, I'm not trying to make my mark on the world. I just want to get shit done and move on. Solnic obviously wants to make something bigger than just a website. He's looking for a bluer ocean to tame. Which is cool, we need all kinds of coders.
What this is really about is that Rails isn't for him. I can accept that. But his rationale is all wrong, it expects Rails to be something it isn't and never could be. Rails is absolutely the best at what it does for the kinds of coders it's intended to do it for. It tracks the state of the art of web development and boils down most of the complexity with user-friendly abstractions.
It's not terribly flexible, but Rails doesn't want to be flexible. It wants to be productive. He wants to go help build the next Rails. Godspeed. I hope he succeeds so that I can go use his framework when it's finally ready in 10 years or so.