Here’s me going a bit further, but I’d be happy with just removing the downgrading don’t get me wrong - but this is what I would do:
There are no keys
Dungeons are unlocked instead. Any given dungeon exists on a variety of difficulties. At first you can see only normal (or +1 if we’re being technical) but more can be unlocked. All keystone levels are unlocked for all dungeons at once.
The timer never downgrades
You run it, you get a timer. +3, +2, +1.
They work the same as today, except of course they unlock the difficulties of your completed timer plus the difficulty you ran in the first place.
There is no way for the system to “de-unlock” a difficulty level other than a new season, effectively removing the -1 timer.
Full reward for however long it takes
Like today, even if it takes 3 hours to clear the dungeon, you still get full loot. You get +0 though - no new unlocks.
Although I might reconsider that. Perhaps +1 should simply be the default with no further timer.
Some class changes
One of the things that annoy me a bit about modern WoW is how resource systems have been drained out of our classes. This often leaves you with super annoying decision points like whether I should use my CD because I’m afraid the tank might not pull for a bit of the healer needs to drink and then I can’t hit stuff and get my CD reduction because frost, etc. etc. and this problem is hugely compounded by the keystone timer system, hence why I mention it here.
I think the game should allow certain CD’s to be lowered by drinking, so if I drink I can more quickly get combustion back, etc. Dungeons should be about beating mobs first and foremost - though of course everyone popping their CD’s at the end of a mob means they’d have to drink for like 30 seconds to get it back, so there’s still a decision point somewhat.
Whatever it takes, we’ve got to get this stress out of M+.