Hard Reset vs Soft Reset
It is my understanding that a "Hard Reset" would effectively wipe out all MMR and everyone would be back to even footing.
A "Soft Reset" will keep your pre-season MMR and rank you similarly to where you ended depending on your win/loss in promos.
Hard Reset would group Challenger players with Bronze players in promos.
Soft Reset would group you with players of similar pre-season MMR.
Can it be explained why a Hard Reset is not going to happen?
It would seem that it would create much more of a challenge on both sides of the coin; Having to compete vs higher tier players; Having to compete with lower tier players.
The road to Challenger should be a "Challenge" and having all the high MMR players from the previous season simply place around the same level doesn't seem to promote that sort of thing.
FYI: I am not high MMR but I would love to try my hand vs high MMR players. It's the best way to improve.