I think if there is a line for third party characters, we've kind of hit it. I think after 4 third party characters and three of the most iconic names in video games, I don't think we can really go any further with it without it becoming silly. Rayman is cool and all, but he is not on the same caliber of the other third party characters. There is also the fact that I think his inclusion would be more polarized. In general, I don't think third party characters should be DLC.
But Sakurai said that this is likely to be the last Smash Bros he'll be working on. And if no one else will take the helm, we'll probably never see Rayman playable in another Smash Bros, let alone any other third parties. Besides, who wants to wait another agonizing 6-8 years for another Smash Bros hoping for Rayman to be playable? It's now or never for us!
Besides, what's the point of giving Rayman three trophies, let alone a trophy of himself, if he's not going to be playable? I'm sorry, but I refuse to believe that they won't add him as DLC just because there would be too many third parties already. The roster has been overrepresented with Mario, Fire Emblem, Pokemon, and there's more than 4 characters each. Yet, people complain about the possibility of four third parties being too much. I would gladly accept Rayman as the final 3rd party addition to the roster and nothing more. People has been clamoring for Mewtwo to return, while others disagree. Now he's coming as DLC. I don't see why they couldn't do the same for Rayman.