it could happen, but the trophy upgrades from 4 where all for veterans too, which are all already present in Ultimate. Call it cherrypicking, but I don't think this is such an argument killer as if we saw something like K. Rool added as DLC during Sm4sh despite his trophy status. I'll remain skeptic on the idea until such an upgrade actually happens.
I'll say I'd be a lot more inclined to believe spirits mean nothing if it weren't for
being absent at the time of the games release from such a list with how many minor Mario mooks that had already. For 3rd parties in particular, I don't think they'd have gone through the trouble of getting whatever paperwork involved to have their spirits if there was any chance they'd make these characters DLC later down the road, when this was clearly planned during development and not some time after launch.