The reasoning that there is no downside in adding the RPG characters and locations to the other games would be my similar feeling on how adding Daisy and Waluigi to other more "respected series" (Super Mario platformers, character-centered games like Wario or Yoshi, the Mario RPGs) have no downside. Some might think that there is all the downside in the world for adding either to newer games in a major way, especially Waluigi (some justified that he can't be added with the effective reasoning being equivalent to "staying in the kitchen"), but I would chalk it up to just not liking them.
Indeed, one can look no further on it when Daisy is added as playable in Super Mario Bros. Wonder (and maybe to a lesser extent, Super Mario Run). Sure, there may be some who are upset at her inclusion, but it was overall a positive thing because people cheered for it! I do think that the volume of superfans (vocally) dampened this a bit, because detractors can still pinpoint these people to make fun of others for merely liking her.
I also feel that Waluigi sort of got this treatment. Yes, it's true that he's not featured in one of the "respected series", but considering he's introduced in what some might consider an outsider series by virtue of being developed by a non-Nintendo developer, he's actually got it quite good. In that, he could very well be stuck in Mario Tennis and that would be it (some detractors might even view it as the preferable outcome, I imagine), but considering that he got into a lot of Mario series without much issue, we can already see there there is overall no downside to his inclusion, and I think he would be fine in any of the aforementioned "respected series".
Not to mention, despite being created for a Camelot game, there were no credits to Camelot for Waluigi's inclusion in other games, meaning that he is basically treated as an in-house Nintendo character unlike Geno (the GBA Mario & Luigi specifically credited Square-Enix despite Geno being a mere cameo).
Thank you for reading.