I think if you boil the problem to the simplest form, the frustration isn't "There's 2 lines", it's "I can see two lines", the users preference is less line spacing.
That's at the visualisation layer, not at the content layer IMO. Because from e.g., the perspective of a plugin who's job is to count paragraphs
this
is
not
identical
to
this
So swapping the button is equivalent to changing the content layer to fix a visualisation problem, which is what CSS tried to save HTML from, and, being literally WebTech, is pretty comparable with Joplins Markdown IMO.