Personally I would find it annoying to answer any interactive prompt and I think that the flag for someone who actually wants to build does not need to be made any more difficult than --build-sdists.
Most commonly I would use this flag in CI jobs where interactive prompts don’t work anyway. There is already enough time spent fiddling with versions in CI jobs so we don’t need to create new unnecessary reasons to do it.
A PEP should not presume any particular UI. The metadata in pyproject.toml can have a flag and a help string but it should not be presumed that those are going to be used for an interactive prompt.