It's interesting to me that people expect that people expect Bootstrap and friends to grow to large scale projects. As implied by the name, Bootstrap's great for getting your new project off the ground. Once you're up and running though, you should have the time (and/or the team) to take care of things in-house.
Why? That's a bit like saying a back end development framework is only a starting point, and you should start working outside of it as soon as your product is mature.
I don't think the name carries the explicit rejection of ongoing use that you believe it does. That you can use it to get started quickly does not mean you can't use it indefinitely; it's often necessary to tweak or hack Bootstrap to fit your specific visual requirements, but for a project that can actually use it without serious modification, I don't see a reason to throw out the convenience and consistency it provides arbitrarily, simply as a matter of course.
But that's just the case. Also back end frameworks give general good enough solutions for problems but they are never optimized for particular use case. If you stay in general framework solution your competitive advantage is often easier to copy so you must make difference with customized / tailored solution. Earlier you start smaller the change is. If you hang yourself to general solution too long switch might be even impossible to make since costs and risk grow too high. And then you are in death spiral when more flexible start-ups begin to eat your share.
Can you give an example of when this has happened? It strikes me that driving your model around being difficult to replicate from a technical point of view wouldn't necessarily lead to good business decisions.
