Short-circuiting is natural for operators (as it already exists in or etc.) and useful for lazy evaluation in:
x ?? calculate_expensive_fallback().
Old broken x or default constructs can easily be fixed to x ?? default without introducing more bugs in the rewrite to coalesce(x, default) which requires editing in three places instead of one.
Chaining is much clearer and less error prone with operators:
(override ?? fallback).name ?? default
coalesce(coalesce(override, fallback).name, default)
The word coalesce is difficult to remember and spell.
A coalesce function can not replace ?. etc. I think?
A keyword-based coalesce operator x coalesce default could be plausible, but seems implausible for ?. etc.
Overall ?? wins IMO. (And is already more familiar from other languages.)