The series will be fine even if they cut 15+ characters, people will still buy the games as long theyre high quality and are fun.
Street Fighter 6 had a lot of cuts and people still think it's a great game
, same thing can easily happen to Smash 6.
In fact both a Smash 6 and Ultimate Deluxe would sell well, because duh it's Smash Bros, some people might prefer one over the other, but claiming one of the two would flop or would be a bad idea because it's not what you want next it's kinda ridiculous to me lol.
Sure some people who don't want Ult Deluxe or Smash 6 might skip it, but that doesnt mean they will "flop" or will be "hated".