Individually, many proposals make sense. Together they are insanity to the point of endangering the future of C++. - Bjarne Stroustrup, 2018.pic.twitter.com/53jehdZfQi
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What exactly is the complaint here? You need to spell out somewhere what it means to behave like a bool, and that's how it looks like. It's not as much an abstraction as it's turning something which would be spec language otherwise into code.
Two things. First, bool works fine and doesn’t need an abstraction. Second, this is a fake abstraction because it breaks fundamental properties of bools, such as short-circuit evaluation.
IMHO c++ stl library should be considered as bad toolset and has passed the point of no return many years ago.
The language features themselves are good (for example, I wish I could use C++14/17 constexpr in my C++11 codebase), but I agree the stdlib just gets worse and worse (and its current implementations like libstdc++ and libc++ aren't much better).
amazing flexibility for user code though
I might disagree with you on some things (like the EGS), but as a C++ developer, I have no choice but to agree with you here.
My C++11 code is already quite slow to compile IMO. I don't want to imagine how slower it'd be if I were to switch to C++20 and add concepts. 
If you currently use SFINAE, concepts might actually be faster. If you don't, they won't (but improve error messages).
What would you write instead for this use case ?(e.g. having a generic function that only accepts bool-like types)
The problem is not even abstractions. The problem is that they are being approached from completely the wrong side/mindset. Irrelevant details and amplified, yet other, more fundamental problems are being ignored.
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