I've talked a bit about this but resetting MMR does nothing except cause chaos to matchmaking. Imagine playing ranked where it was a true coinflip on whether you got previous plat or radiant players and if the other team had bronze or Iron. I have only ever seen one game do an MMR reset; even games that say they have are not telling the whole truth(in fact, they are not showing real MMR, but an altered one).
Part of the reason is those skill systems that determine MMR take lots of matches to get an idea of where you belong and opponents with various MMR levels to test you against. You are destroying years of data building that helps the system rate players against each other by resetting MMR. If you were to take chess as an example, it takes players 1000's of matches to reach their "True MMR/Skill". Imagine resetting a player before they reach that mark. Now you no longer know how good that player is, or any player for that matter, and now you need each player to play 1000's upon 1000's of matches first to fill out the MMR bracket, and then AFTER the MMR ranges fill out, players could start their journey to find their skill.
Now the above point is just about accurately predicting skill. I know the response to this is, "Well, the system doesn't calculate skill right, so just reset everything and start over." If the system is flawed, it won't change the results. It's not going to make it easier for you to climb; it will just make you play more games to get precisely right back where you were. MMR is a math equation, and you are a number in that equation. If we put you, the number, into that equation, you will get the same result. The equation or the input has to change; resetting doesn't change any of those things.
Not only have we done the research on this, but many studios that have matchmakers also have. We recently investigated the statement, "Start an Alt if you are hard stuck, and you will climb easier." It turns out this statement was not true; you have less than a 40% chance to end up higher rank than your main at any point up to the same amount of matches played. Even when we did see people reach their main rank, they stayed at the same rank as their main or one above in most cases. The most common cause of someone climbing higher on an alt was more time dedicated and grinding that account actually improving, which could have been done on their main. There is also probably a sub-conscious factor, how you play or who you play with, that can heavily affect this. We learned the system couldn't be tricked unless you improve, and there are no loopholes (which we've validated before but wanted to check this common statement from social media).
If we stop talking about how things are calculated and look at MMR for what it is, it also does not make sense to reset MMR on a philosophical level. MMR is just a ladder with fancy math; all it does is place everyone on a giant ladder from top to bottom. If you beat an opponent, you climb up the ladder and they climb down. If we reset MMR, it wouldn't do anything because it wouldn't change the opponents you can or cannot win against. If you aren't climbing, it's because you can't beat the opponents at your spot on the ladder, and you can't rise above them. If we reset the ladder, everyone would go back to their spot. You would end up winning/losing against the same people because your actual skill hasn't changed. Resting MMR isn't magically going to make it so you can beat the people you cannot now because if you could, you would be higher on the ladder and have a higher MMR.
So if we reset MMR, nothing would change, and the best players would have a great time stomping everyone getting back to their intended MMR while anyone below them would get crushed and have a bad time.
I wanted to answer your comment below on why reset a milestone and not MMR. We have a hybrid system, which uses a rank sitting on your MMR because it forces you to play matches and validate your rank. We need a system that can work for players that choose to play five games of ranked one act or players that play 500.
Lets say Episode 1, you played 500 matches and got Gold 2, and your MMR is at Gold 2. You then take a break and come back in Episode 2. You have a couple of good games a couple bad ones, but it's only five matches, and your MMR is still around Gold 2 after those games. We need you to play more matches in order to say, "Yea, even tho you took a break, you are still gold 2". We could set your rank at Gold 2, and that's it, you're gold 2. But what if we got it wrong? What if you just had a good streak, got carried, etc., but you came back rusty, and your skill is actually Silver 2. Now your experience is not only losing games until your MMR corrects, but the system will also push your rank down to Silver 2, and it would feel not very good. To top this off, you would get a Gold 2 reward for that season, even tho you indeed were not Gold 2 anymore.
We reset ranks for all of these reasons. It's not a good experience to be placed at your actual MMR and have the system be wrong. When soft resets happen, it forces players to play matches to prove they deserve that rank; it would not be fair to hand the super bowl trophy to the same team that won it last year would it? At the end of the day, if you can't get your rank back, you don't deserve it in the current season. The community gets better, average skill in ranks change; for example, an immortal in episode 1 is probably closer to a Plat now. It also makes it so if you earn a ranked reward in that season, it has meaning. Someone isn't just going to show up to check a box, play a couple of matches, and because they were once immortal coast at that rank for months/years(and get rewards for it).
I've explained this better in other posts, but I wanted to answer your question because it's not always easy to see why we do things this way or understand our thought processes. I hope this helps, and I know ranked can be frustrating if you don't see the results you expect. Ranked will always have tweaks, the game will be evolving, and the community will be learning together. Even if you are frustrated now, hopefully, a change in the future will help solve some of your pain points. Good luck, and I'll try to answer any other questions I see pop up!
devtrackers.gg