Trust me, with all the characters in this game, you would probably despise true All-Star mode after the third or fourth playthrough.
In all seriousness though, I will agree that Ultimate felt like the easiest game to 100% all the challenges, and World of Light, while I personally enjoyed it, did drag on a bit near the end, but the first playthrough was well worth the experience, but there's nothing to really gain from a second playthrough nor New Game +, and once you collect every spirit in the game, there's no real reason to ever go back to the spirit submenu outside of the DLC spirits they appear to be slowly adding in via updates (praying for some Persona themed spirits).
I personally enjoyed collecting spirits, and thought them replacing trophies was understandable given the sheer amount of universes and characters in Smash, but I do miss the flavor text and descriptions they gave to trophies, and was one thing I was bummed out that they omitted.
I know side modes like Home Run contest has its fans, but I can understand their omission considering the changes to the mechanics, and its obviously not something people really messed around with in a common scenario; if you're with friends, you're gonna be playing the regular Smash mode, and if you're alone, you're probably either doing Classic mode, World of Light, or online.
If there are some things that I do miss that would be reasonable requests, it would be a slightly better race to the finish bonus game (the one in Ultimate wears out its welcome after the third time, and Melee showed that you can have a single track while being dynamic with the difficulty and still never getting old after multiple playthroughs thanks to multiple pathways and the different time limits for each character) and coin launcher from Brawl, which was one of the best sidemodes in Smash ever IMO.
But yea, I am content with Ultimate's content overall, and I appreciate it over Smash 4 solely for having cleaner menus and a cleaner presentation overall, but I do think it's very easy for a completionist to clean out every corner this game has to offer, and unless you're a competitive player who is practicing to get better everyday and is trying to grind out as many characters as possible, there probably isn't too much replay value once they finish World of Light, clear out the challenge board, and obtain every possible collectable.