I think it's more that it doesn't suddenly help his presence in Japan become way larger, which is part of the issue.
It also doesn't necessarily mean he'll start getting new games where our only related examples are DK who got none after.
Basically, people are pessimistic due to a precedent set by the previous DLC; whoever is in it doesn't have their presence changed(in other words, they don't suddenly get a new game or anything like that). It's a big deal in a sense of Rayman being used, but there's no results that show this usage will amount to anything.
This isn't saying optimism is blind either. It's absolutely great in this situation, and I'm glad to see it gives the fans something fun to talk about. I do, as I said before, hope it leads to great things.
To say the least, I'm raising an eyebrow at it because there's not enough reason for me to believe it'll definitely help his situation. But I think it's plausible. I'm not highly optimistic, but I'm not outright pessimistic either.
I hope this helps explains why some aren't jumping at the idea being "definitely good".