As a Smash player myself, Ultimate feels more "button mashy" than just about any other fighting game at this point.
And no, Smash isn't the only one left pulling in numbers. 20 million units sold is unprecedented for a fighting game, thus you're implying no other fighting game ever has pulled in numbers? Not Street Fighter 2, not Tekken 3, not Mortal Kombat X? Ok dude.
When someone says that Smash is the only fighting game left that can pull these numbers, I don't think SF2 (released in 1991) and Tekken 3 (released in 1998) do much to refute that point.
That said, from what I can find SF2 sold 15million, Tekken 3 sold 8million and MKX sold 11million, making Smash Ultimate the best selling fighting game of all time with over 20million sales.