Think about Last Wish but with matchmaking, with those same folk that can't even play gambit effectively. It sounds like an absolute nightmare already. And when people start getting hardly any completions from raid matchmaking, you can bet your are they'll complain about the difficulty.
Why does it matter? If you want to LFG you can, regardless of whether or not matchmaking exists. People hardly get completions from Raids now, because the extraordinary hoops players have to jump through to even get one started. What difference does it make if completions are low due to ineffective parties?
You can also let people at their own matchmaking parameters. If you don't care who you play with, you open your search. If your picky and don't mind a longer wait, you limit your search to people with similar ranks, or who have completed raids that you have.
Also, let's not pretend the LFG eliminates possibility of getting shitty teammates.
What happens when you're three hours in and just about halfway and someone gets frustrated and quits out? Do you match-make people in halfway through a raid, if so what about those going in wanting to experience the whole thing from start to finish? Checkpointing could work but would populations for each part allow for a quick replacement?
Genuine questions as it's really not as simple a solution as people make it seem. Putting LFG in the game would work but adding standard matchmaking to raids would make for a potentially terrible/impossible experience presented as a choice for a game mode.
Not being able to participate in Raids because you don't have the time to coordinate via LFG nor the online friends to play with is a makes for a terrible/impossible experience presented as a choice.
You know what has been historically a great way to make online friends with similar interests and experience? Matchmaking!
As I said before, the existence of Matchmaking doesn't prevent you from using LFG. And LFG doesn't prevent someone from leaving unexpectedly.
