If Nintendo does never new IPs, how come they have a lot of them. Just take a look at Super Smash Bros vs. Playstation All Star Battle Royale. Sony had to deeply scrape the barrel for IPs they could include and still have a much less remarkable roster. I don't even compare to Smash Ultimate, just compare with Smash Brawl, which was at a similar time. Nintendo could whip up easily more remarkable IPs to fill the roster than Sony. So something says to me, that actually Nintendo isn't that bad at creating IPs.
I did as you suggested and took a look at the Smash Bros roster but I don't really understand your point.
If you remove "dark" versions and third-party characters, it looks like most of the characters come from the same "universes". I'm going to take out characters like "Mii Fighter" and "Wii Trainer," too. If Sony had gone that route, for example, they'd have access to all the Gods from the God of War series, all the Uncharted characters, all the Killzone characters, all the Ratchet & Clank characters, all the Sly Cooper characters, now all the characters from the Spider-Man game ... well, you get the picture.
Not quite the same situation.