Even if 13-15 players obviously can't run a 25 man raid, what stops them from doing 3x 10 man raids instead? Blood Legion does 5 full clears on opening - that means that every raider has anywhere from 3 to 5 alts, capable of clearing normal modes. If the same were true for 10 man, then they'd be on par with paragons one 10 man, one 25 man clear.