I feel like MM just tries to get to know players a little bit too hard to the point that it becomes ineffective. As a VG player my cas MM times takes like 5+ minutes because no other VG player is either playing casual or no VG player is playing at all. This results in the MM system to look for the next best thing: POA players. This slowly goes down and down and down and down. They personally matched you against that individual who is also in queue and it begins assigning to basically a full lobby.
So basically casual MM supports the more casual audience as higher tiers have more wait time and lower tiers have less wait time because there are other low tier players playing, trying to play in an environment that is stress free. The only time the amount of high tier players waiting in queue would be when a new hero is released to the week after the new hero is released. Or events like the Idris one. High tier players would party up and wait in casual MM for 10+ min and crush their t4 opponents and picking up a win OR (since there is supposed activity of high tier players) they would run into them instantly and basically have a sweaty ranked match for win on their tracker. A little bit off topic but that is one of the reasons why the game isn’t really about skill but instead partying and steamrolling the opponents who are hopefully not in a party.
Ranked MM is fairly active within high tiers but if there are people playing in “off times” they will do the same process as casual MM. The MM sounds great on paper until there is this amazing thing called “parties” and SEMC will not be able to restrict the way they made parties work in 3s. And that is due to the player base. SEMC just has to make “soloQ” “duoQ/trioQ” and “5-man.” High tiers will greatly suffer from this. They can also try to discourage partying but doing some dividing of the ELO among members so people only party up when they about to get down and play for hours and hours. That would remove boosting from the game, mostly.
TLDR: From what I have noticed MM mostly uses a tiered system that continuously moves down to get you a match you can play. Partying makes MM go kookoo and allows SoloQ players to be in a disadvantage. I look at leaderboard sometimes to see the names and I can point out who’s boosted and not boosted, and a bunch are, oh I will never forget how a VG Silver Varya was losing to a POA bronze Celeste in lane (who reached only POA Gold) and needed a pro player to carry the game.