Can there be an option to reset MMR every season?

if you mean give everyone the same mmr, 0 or watever then things are gonna be a hell lot more sporadic than it already is cuz diamonds will be matched up with bronzes for a week or 2.. or 6 till everything settles ;l

They do a soft reset every season.

Completely resetting mmr would only encourage the same issues that we now see at unranked and pre-level-30 games. If you have friends just starting to play league now and getting into player match-ups you'll realize how incredibly unfair that bracket can be.