To offer a counterpoint:
DLC is also made to continue advertising the base game and keep it in the limelight. And companies are very often interested in expanding their "net" and bring in more money directly or indirectly. One example: Fortnite did feature a Thanos-centered game mode last year. In return, Fortnite got exposure in Endgame. One can even view Banjo with this lens since Nintendo's dealing with Microsoft, a direct competitor for all intents and purposes.
Of course, keeping interest high among those who bought the base game is also a priority; but I doubt that's Nintendo's only goal. One reason I've been bringing up the Switch's
upcoming Chinese release a lot lately is because Nintendo's entering a new market and the Switch's distributor in China is Tencent. Now, there are a couple things I should emphasize:
A) Tencent is a massive company. Waaay bigger than Nintendo, at least approaching if not equalling Microsoft / Google. Their WeChat platform is basically the Chinese Skype. Also they (co-)own lots of games, most notably League of Legends.
B) There have been four games sent in for approval: Breath of the Wild, Mario Kart 8, Super Mario Odyssey and Pokémon: Let’s Go. Smash is not going to be far off.
C) As such, I wouldn't be surprised if Tencent brought Smash up to Nintendo. Now, I strongly doubt it will be for this Fighter's Pass: there's no set release date just yet in China, and there have only been a very loose rumor about a LoL mobile port. However, as I talked about in the RTC thread, there's already a company that benefits from the Chinese release: SNK. Both Tencent and Nintendo just so happen to be on
good terms with SNK too.
All in all, don't be surprised if SNK (most likely) or the surprise Tencent pick makes it in. Either of them being in Smash would help sell the game to the Chinese market - take note that Nintendo's putting in quite a lot of effort into the Chinese release evidently. I mean, Nintendo's partnering with a Chinese megacorp to get through loads of government red tape.