i mean who's left? some of the more prominent choices are ATs or Mii costumes. we got people who fervently believe that spirits are an instakill so by their logic those are gone too. and i'm pretty sure that every nintendo character with some sort of official art will be a spirit, so if that theory pans out that's a lot more candidates down the drain. upcoming games? what characters can you realistically pull from luigi's mansion, metroid prime 4 and animal crossing that are actually pausible to include and aren't spirits? not that many. FE, Pokemon and new IPs that aren't represented in smash are some of the last viable first party choices but after seeing so many obvious choices get deconfirmed it's not like those are guaranteed to appear. so yeah a heavy third party lineup is very likely assuming spirits are deconfirms.
The only reason people are assuming third-parties will consume most of the DLC is because (1) Steve and a SE rep are the two most prominent characters mentioned by insiders, and (2) a lot of popular first-party characters are Spirits. Neither of those directly indicate that we're mainly getting third-parties.
It's WAY too soon to assume anything, especially since we haven't even seen who the first character is.
plus they will probably sell way better than most of the stuuf people support here so there's that incentive for nintendo as well.
You do realize that Nintendo has to pay for these characters, right? Even if the third-parties really did sell better than first-parties, the money Nintendo would have to pay would likely make up the difference.
Unless it's being used as a stepping stone for future collaborations, the third-party company benefits the most from getting their character in Smash, not Nintendo.