Why? It’s still the good (bad) old pimpl pattern. It just got a bit shorter. When reading you dont even need to grok “std::indirect”, you see the word pimpl and you know what’s going on.

Because I will still encounter people who use the old of writing pimpl. So I now I'm forced to remember the old way + new way. People aren't magically switching to use std::indirect. I bet that even in 10 years, half of projects will still be using the old way.