Why we should we not see a competitive Rebel counterpart then? I think that would hurt nobody and diversity of lists is a good thing.
Because for some people the loss of factional identity is a bad thing. If the difference between the Imperials and Rebels in terms of game play is lessened or goes away completely this can hurt the game.
I'm not sure I agree, but it is a valid opinion.
Just because the Z-95 doesn't work well as a 6+ ship swarm doesn't mean they don't have a place in the meta or even in tournaments.
Besides is 6 ships any less of a swarm then 7? Exactly how many ships do you need to make a swarm?
The Rebel's don't, nor should they have, a swarm dynamic for the same reason a wizard can't wear heavy armor and carry a battle ax in fantasy games. Diversity is what makes the game interesting and challenging. The player must choose and make compromises. Mastering those choices and compromises is what make a game fun and repayable.