I think it is objectively wrong to replace 20man with 10man (on current mythic mode instances) because the game has 12 (unique) classes so it REQUIRES a gametype with more than a dozen of players to reach its full potential and it must be the same size per instance to keep the competition on equal ground.
However: because of the SAME reason: 5man is too limited because the metas are extreme (noticed how all world records have a frost mage lately?) so: a new 10man (on their unique dungeons) might be good.
PS practical consideration about it: because life is short: they should reduce 5mans a little in order to do that; for example: instead of 10 dungeons have 8 and have the other 2 be 10man.