This is so incredibly true. Honestly, if K. Rool had been DLC, I would have instantly dropped my money on him and gone out of my way. I wasn't even so sure about how soon I'd get Ultimate, but once K. Rool was revealed, I was like, "I HAVE to get this game ASAP." Other characters like Banjo or Geno would have given that effect, as well, and will do so for Fighter's Pass if one of them ends up in (which is like, nigh-0% chance lmao).
With that said, Erdrick, if he is our rep, will definitely have a neat move set, but the reception of who the character is will count either positively or negatively for him. (Mostly the former for the east, and I'd wager more of the latter for the west.) Corrin doesn't seem to be so well-received by too many people, as far as I can tell, yet his/her moveset is fantastic. They're a dynamic and interesting character. With that said, though, I didn't really care and never bought them as DLC.
Erdrick is such a weird case to me that it's like... it's a wait-and-see game. While I think he definitely deserves the slot, I'm pretty meh about him and I wouldn't buy him, no matter how cool his move set is. If a character doesn't have staying power for my particular interest beyond a move set, I'll just skip them out. That's just me, though.