In the rationale for this that I can find [1], a maintainer says the following:
> I'm inclined to say we should do it, even though it will cause some disruption.
They also say an alternative is to "accept the status quo", which is exactily what they should be doing. I can't find maintainers giving a compelling reason not to support this status quo of `long-description` as an alias to `long_description` besides "simplifying code." Code simplification should never take precedence over massive breakage of compatibility.
[1] https://github.com/pypa/setuptools/pull/4870#pullrequestrevi...