The game objectively needs 20man hard mode raiding because it has 12 classes so it must have a gametype with more than a dozen classes to reach its full potential and it must not flex on hard mode (mythic) because it can not be balanced well otherwise because of various technical issues.
However: for the exact same reason: 5man is very class restricted and inevitably creates extreme metas (noticed how all world records had a frost mage lately?) so a 10man gametype is required if you want a mid point which is not 20man but an improved 5man at that.
PS Because life is short and because 10man mythic would need its own dungeons to be balanced for competition: 5man dungeons should be reduced from say 10 to 8 at launch and have the other 2 be 10man or another variation of that sort of thing.
As long as that 10-man content has its own gear progression path that does not affect any other form of content, sure, whatever. (While there, please make 20-man raids the same too, thanks.)
Definitely. The system has built-in at its core that the 10man dungeons should be unique (and probably that would mean the 5man dungeons should be reduced by 1 or 2 but that’s a separate issue).
We have to respect that hard mode raiding/dungeoning can not be done with flex mode because you can’t balance it.