Except if they take that route, they'll have more time for more newcomers (about 15, as Brawl and SSB4 already showed), more new stages (around 20, compared to Ult's grand total of four) and brand new modes, gameplay mechanics, etc.
?
Ultimate added: Snake, Squirtle, Ivysaur, Wolf, Ice Climbers, Pichu, Young Link, Inkling, Ridley, Isabelle, Incineroar, Simon and King K. Rool.
That's 13 characters that weren't in the previous game. The usual amount of newcomers more or less.
(+ the echo fighters. Ken and Chrom definitely do enough to feel like their own characters and Dark Samus has a ton of new animations, but I won't count them)
(+ the 12 DLC characters but let's say they don't count for some reason)
It also added an absurd amount of stages. Making most of those legacy ones is just a choice. They made a ton of stages and they obviously worked a lot on it. The Melee, Brawl and 3DS stages in particular all had to undergo a huuuuuge makeover or probably were straight-up remade from scratch.
It also changed a lot of gameplay mechanics compared to Smash 4. Everyone is faster, balloon knockback, universal jump squat, directional air-dodge (and only one air dodge per jump), perfect shield works differently, you can keep smash attacks fully charged.
Single player modes get more subjective but in my experience Ultimate has the best single player in series history.
It offers for the first time a (somewhat) reliable score attack system that makes everything extremely replayable, and it also offers 1400 unique, tailor-made fights.
Really can't see what the big price Ultimate had to pay to do EiH would be... If we're doing ad hominems, just say you miss Subspace Emissary.