Consider this. Your 450-460 gear running 5s. was actually more powerful than the dungeon dropped in 5s. 5s dropped 447 loot.
Now consider that a 10 last season dropped 454 item level gear. And this seasons 0 which is (last season’s 10) drops 493 gear.
If you were running 10s in 450 you probably need to be in 485 or better this season to do 0s.
The difference is a lot of people are going into 0s in your same shoes and their overlooking the need to go upgrade heroic gear to get into the mid 480 range to be where they were last season.
Their getting the new fancy 476 heroic loot and are like WOOT im ready to go. Their lower ceiling players and were relying on gear and they don’t have the equivalent gear this season before facing off against the 0. Worse yet its there’s is/was significant number of players doing this which compounds the issue where you have a lot more people under geared.
Lastly consider last season you had a lot of people running lower level mythics for fun grossly out gearing it. It wasn’t uncommon to see folks in 480 gear running keys in the 2-10 range (I experienced this having leveled through M+ only the last three weeks of last season).
Again not supporting this change they made but explaining the path to overcome it.
And people should expect to spend as much time as they did getting to +10… in Heroics now.