So consider all the lines of code in Bootstrap that aren't in your hypothetical framework. They either do something or they don't. Let's assume the former because the latter implies incompetence and there's too many eyeballs on Bootstrap for that to be plausible.

So your question can be reformulated as "Bootstrap contains functionality that I don't think is needed".

If you agree that's a reasonable restatement then you can start a much more fruitful discussion if you could give a few examples and your reasoning behind why each one isn't needed.

What are you claiming the extra "thousands - 20" lines of code are for? I'm curious to know whether I need them or not.