This seems much worse than it is, especially without context.
You still have access to every mission, every strike, every raid, every crucible mode, every crucible map, and every item you previously had access to. You don't lose actual, tangible content.
What people are complaining about is that, in Vanilla Destiny, you lose modifiers to those strikes and missions meant as progression content - "Heroics" and "Nightfalls" - which add difficulty to that same content in the name of progression. That progression is meaningless to a player that can't hit the level cap anymore and thus can't equip any of the rewards.
Ignoring the fact that levels don't work how they used to, nor do the "Heroic" and "Nightfall" modifiers, it doesn't make sense to have low level progression content because then all the high level players would just run that instead. You also can't out level content, so its not like vanilla Destiny is any easier - its the same content frozen at one difficulty.
A lot of the things listed in that Reddit post have been reworked entirely and "removed" to instead become something else to all players, not just those with TTK. I think anyone actually playing the game would agree the trade-off were worth it.