static asserts do not replace tag dispatch and SFINAE as they don't affect the overload set, the instantiation has already happened at that point. However, concepts will do this in, probably, c++20. For now, is_detected is like a concepts lite that allows for a big simplification.

One thing to remember though is that there are two things at play here. The library writers tools, like TMP, and the users. The users can do a lot and never worry about SFINAE/tag dispatch. The library writers use it to allow the users of the library to have a potentially very simple experience.

Also, Greenfield C++ can be way different than pre 2011 C++. The cost of things that people would work around has gone way done to zero in some cases. Like returning a value from a function is like constructing it in place at the caller. So that takes away a need to use the heap to allocate it or to copy it when you return it.