Hmmm, I don't think all ranked games should be tier-locked if I'm honest.
The reason for this is that if the games are dependent on MMR like it is now, you gain an absurd amount of points and lose very very few if your MMR is high enough.
Which in turn makes climbing alot quicker if you keep a decent win ratio, and also enables players to climb with even just over a 50% winrate as long as they grind it out.
If you were to make all ranked games tier-locked then it means that LP gain and LP loss is static.
Losses will affect you more, and players that aren't too consistent will go up and down, up and down, all the time.
The way it is atm is smoother to climb.
So in a way we have to be grateful for this system.
But I do believe, like we said, that at least in promotional matches we should be matched with players close to our current division/tier, and not based on MMR. Because it doesn't make sense to play a promotion match to get into Gold (random example), while playing on a much higher level simply because of your hidden MMR.
It's not logical in the slightest.