We all know some of the public statements about the future of Smash have seemed suspect before, but Sakurai has been pretty consistent and specific this time regarding the infeasibility of expanding like Ult did; he's cited the similar system architectures of the Wii U and Switch as well as the ability to take the same team straight from 4 as reasons that Ult occurred.
But again, it's more than Sakurai's statements. In the first place, we didn't need him to tell us those two things in order to observe that they would be major considerations.
In addition, do you really want to bank on essentially indefinite linear growth? Even if you do, numbers still don't add up. From 64 to Smash 4, the base roster numbers went up by about 12 or so per game, on average. Ult had a spike for the reasons outlined by Sakurai. Add 24 or so characters to Smash 4 base, and you still don't get Ult's roster number. Add 12 to base Ult's number, and you still need fairly substantial cuts to make room for newcomers. So even in the most optimistic "linear growth" scenarios, you still need more cuts than we've ever had before to accommodate a reasonable number of newcomers. And again I don't expect linear growth to indefinitely continue, and even if it does we may be talking linear growth from Smash 4, the last ground-up game they made, rather than growth from base Ult.
Factor in other potential trip-ups like third party licensing and yeah I just don't see EiH 2.0 happening.
Lastly, when Sakurai says he doesn't expect an increase like Ult had, he means just that: likely we won't have a game that expands on its predecessor as much as Ult. He's not outright implying an increase will happen; maybe if the next game cuts down the roster some, then the game after that could see an increase again, but that increase likely wont be the increase we saw in Ult. I think that's all you can reasonably take from it.