"They can do it again" isn't the argument you think it is. Especially when Snake didn't come back in 4. Clearly they always cannot do it again every time. We know that for a fact.
It's always a bad argument. Circumstances change. This is an actual fact and why things change when making a new game. Development costs can change depending all kinds of context and circumstances. Everyone Is Here is very unlikely to happen again as is, as the circumstances that led up to it aren't able to be replicated, but moreso how much extreme licensing it'll cost. Our only realistic hope is the other characters being moved to DLC to get everyone back. And that's not the most likely outcome, but the chances are far higher than having 80+ characters in base. It's a lot more money than is expected for a base game budget. And even if somehow it could magically happen, there's a damn good chance we'll have very bareboned other content due to no real time to implement it yet. Sure, it'll (likely) be free DLC later, but it might not work well this time. What made things work well is they didn't plan DLC at all till heavily into the development period. All the DLC characters were a later concept, by Nintendo too(though only most of the choices. PP was the only outright Sakurai choice, respectively).
It's just completely different from other kinds of games. Which many aren't 3rd party guest-heavy to the point of being hard as hell to license. Honestly, Sora not being DLC alone, for a good example, would be extremely surprising. Sephiroth and Hero to a lesser degree. Either way, returning everyone is a colossal task in more than just workload. Though that said, a Deluxe version has even less chance since they'd have to pay the entire licensing fee all over again for an updated port. It's far more worth it in the end for them, especially for sales, to make an all new game to get more people interested. It's already going to cost 70 base for us, and then you have to factor in later DLC. With little new content due to being a port, the price is pretty extreme now. Sure, it sounds like a good deal... but you paid for nearly the whole thing anyway. Not all ports work that well, and let's keep in mind that a lot of ports also only work often because they're not based around 3rd party licensing fees. It's not a matter of Nintendo simply affording it, but getting an actual good return, which... isn't very strong when they'd be losing a lot of money trying to port it over. Not enough are guaranteed to be interested when DLC is going to make the costs worse(you didn't think the 3rd Party Mii Costumes were going to be in base, did you? It won't happen. Companies twice made sure of that. That's a guarantee).