There's a lot about this game that could be better, but it's nice to know that they improved at the single-player content substantially from 4. I haven't gotten very far in World of Light because I honestly just haven't found it that fun, but even unlocking all characters through regular Smash took me about a week (and I was playing a decent amount even), and the Classic mode is a surprisingly convincing time waster considering it doesn't seem like a ton of development time needed to be put into it (lots of great easter eggs and nods as well as some very M-rated jokes make it a treat, and the GSP rating helps, then again I'd just prefer a scoreboard).
However while I will wait until finishing World of Light to give my final opinion on it, just reiterating how long it is isn't doing it any favors. I've heard it drags on for people, some stating it's just the last few hours, others saying it's more than half it's run. I don't know if they'r right but if I had to choose between a 20 hour campaign that wasn't very good and a 10 hour campaign that was great, I'd choose the latter.
While Ultimate does suffer a little bit from a lack of truly new content, to imply that this was only developed in one year is crazy. Pretty much all of the maps that were made before 4 were given a huge face lift and even some of the ones from 4 I'd imagine have been slightly improved. The characters are for the most part extremely balanced and a lot of the newcomers have some of the most interesting movesets in the game. You wrote in your OP that "Sakurai was probably only given 1 year to make a new smash and he chose multiplayer quantity over quality/replayability", which to me sounds like the problem that 4 suffered from, not this game.
Would I like Ultimate to have single player which appeals to my personal senses more? Absolutely, and truthfully I think there's a lot of heavy criticism that can be given. But as long as games like Smash 4 and Ultimate are given a 2.5 year development time, we're never going to get the "Ultimate" Smash Brothers ... there will always be things the previous games did better, because development times are only going up and even with a bigger staff on hand there's just so much you can focus on at a time. I would love the next Smash Bros to be developed over many many years, but at this rate I don't think that'll happen ... it's not necessary to get the kind of sales the series has gotten.