Not really

Splatoon 1 did keep things fresh with new additions, but it was super barebones when the game launched. Smash fighters is what makes up this game so building up to 75 would be a huge drag for those that want all the characters.

This is a horrible Idea. Ultimate's main selling point was "Everyone is here". They would have to take that away. It's not like Splatoon and Arms had the content, but kept it away for dlc down the line. New content was being made after the game came out

Cutting down on characters wouldn't have necessarily sped all the other sections of the development up.

The way video game development works is they staff people for different segments of the game and usually in the final stages they're polishing stuff. It's almost certain the character artists and animators, and any engineering resources (and xml scripters) were entirely different staffers than the rest of the team. The only staff they'd have had in common was QA, and cutting out a few characters would have saved them days, not months.

No way. I think the releasing it this month during the holiday was the right decision. Sure, there's a lot of characters to unlock, but it won't take too long to do so. :)

Despite Sakurai saying this game was not a port from the Wii U version, it seems to me as though much of the groundwork was taken from that game. Thus, a large part of the development time was indeed creating characters and balancing them. I get your point, and for many games what you're saying is probably true, but I don't think so for Smash. I don't think they can pump out a new character every single day as your post implies.

And on a side note, it looks like my suggestion for getting a dark version of TinyMCE to match the dark theme seems to have been implemented. Thanks to whomever made that happen! :)

The free DLC from Nintendo hasn't exactly been an amazing system. I don't even want to get into that again, but needless to say I don't like it, it's terrible. So, no thank you. Focus on a few characters you know from the previous games, or a few characters you like, just like everyone else does. That will help you learn the game faster.

No, the Holiday season is when Nintendo sells the most games and sells the most consoles.

If they did that... the title would be super smash bros: kinda ultimate until by the end of the year

If Smash Bros. Ultimate worked in the Splatoon/ARMS-like DLC system, it would've been hard to advertise the game as "Ultimate." Plus, the big marketing push for the game was "Everyone is here!" That's over 60+ characters already.

Plus, Sakurai does not really roll with that type of content system. He's usually one, as noted from previous interviews, that wants to provide the best game he can make, thus, slowing releasing content akin to Splatoon/ARMS is not his style.

Ultimately. no pun intended, he needed about 2.5 years (actual development started in February 2016 as per his latest interview) to complete Ultimate.
https://www.sourcegaming.info/2018/12/12/sakurai-tells-more-details-about-smash-ultimates-development/
So Summer was unlikely for him to release the game as advertised.