I wasn't going to jump in on this particular conversation because I'm tired, but I get the feeling I'll forget about it if I don't.
I think there are ways of looking at the Smash Ultimate roster, but something that cannot be denied, in my humble opinion, is the thematic energy this game gives off.
Sakurai has referred to it as the "biggest crossover in gaming history"...he has expressed excitement about the sheer amount of IPs and series crossing over together, saying that "there's a good chance a crossover of this size would never happen again"...you have Reggie mentioning the DLC of Fighter Pass 1 in the context of, "Mr. Sakurai...he's a student/scholar of gaming as a whole" (emphasis on *not just* Nintendo), then you have the packing of the game itself "Gaming icons collide".
Sure, Smash started as a Nintendo crossover, and grew into a larger Nintendo crossover game. It's still a Nintendo crossover game. It just isn't only a Nintendo crossover game. Some might even argue, in fact, that some of the 3rd party IP present in Smash are closely tied to Nintendo, and therefore it's still a Nintendo crossover. However, let's be real...sure, Simon Belmont and Mega Man are nostalgic in terms of those retro gaming years. Sonic is Mario's oldest rival, but still SEGA. Pac-Man is cartoony, but still the Namco arcade legend. That said, despite Shin Megami Tensei, Metal Gear, and Final Fantasy having Nintendo roots in one way or another, Persona 5, Final Fantasy 7, and Metal Gear Solid all certainly have a much stronger connection and presence on non-Nintendo consoles in the overall zeitgeist of the industry. If it were truly only viewed as a Nintendo crossover, I have to believe that particular picks would have never been made, or alternatively, been pushed aside in favor of other 3rd party characters that appear to have a much more "Nintendo" audience, such as Phoenix Wright or Professor Layton, or perhaps even Cooking Mama.
There's also the business aspect to things. Including more 3rd parties in Smash, as well as cooperating with those IP owners to get more IPs selling on Switch is good for Nintendo's bottom line, especially if it means selling more Switch consoles. Working to get an IP like Devil May Cry and Switch? Great! possibly pairing that with a character like Dante? Also a great business move! Does it mean it's guaranteed? No, but it's anything but a bad business move.
Nintendo could continually push their own IP as DLC, but in my humble opinion, I don't know that there's as much to gain. ARMS stands to benefit from renewed interest and a breath of life, which a pick in Smash certainly serves as. Any other non-veteran/non-Piranha Plant Nintendo DLC pick was promotional in nature, and from Fire Emblem...up until now that is. Otherwise, 7/9 DLC picks have come from other companies.
In that sense, Nintendo has largely focused on IPs from outside sources for DLC. It's not a stretch to see why, either. With a mascot crossover as large as Smash has become, and simply based on the fact that we probably won't get a crossover of this size any time thereafter, it seems entirely plausible that they shoot their shot now, and truly cap off the "biggest crossover in gaming history" with making it even larger.
It isn't just DLC, however. Over time, it's become quite clear that a large focus of newcomers has been placed upon 3rd party fighters. We started with 0% of 3rd parties for two games, ~5.5% in Brawl, 10% in Wii U/3DS, up to 19% for Smash Ultimate. The 3rd party presence, from Brawl onward, has nearly doubled in every game. There's certainly been an expansion upon the presence of 3rd parties, but it's not some of "invasion". Nintendo struggled with 3rd parties for years, and now are finding some consistent success with the Switch, not just because they have excellent first party software, but because 3rd parties are actually showing up.
Smash Ultimate is definitely a Nintendo crossover. But the bigger picture, I think, is that it brings some of the gaming industry's best characters, and brings them all together. Smash Ultimate wouldn't be what it is without 3rd parties, and I think that's a-ok. Nintendo is attached to the gaming industry, but it isn’t directly attached to every single amazing gaming character, which truly shows the value of 3rd parties. I don’t really see how Smash Ultimate isn’t both a Nintendo crossover AND gaming crossover.
Do note- it’s not just a “gaming crossover” because of “hype” or “marketing”. Sakurai, the dude folks love to death, mercilessly harass on Twitter to add their character of choice, the dude who spearheads development, has called Smash Ultimate, or referred to it as a “gaming crossover”.
Frankly, it's a dream come true for me. I say bring in more 3rd parties!