I think its a combination of things. But the main reason pros are quitting is because the game has run stale, you can't create content much anymore unless you run with patch updates and discussing that- meta changes etc. New weapons are scarce, a new map might help things. The servers are horrendous too.
Pros also can't pub stomp like they used to in pubs as most of their matches are pretty tough. Apex listened to them and allowed them to play with guys with k/d's just above 1.2 now, and some pub stomping is happening. But season 3 and 4 basically made them run for the hills.
The big reason the majority of the players (non-pros) are bailing on apex is sbmm imo. The majority of players have a k/d in the 0.7 to 1.7 range. Only the small percentage of bots have their own lobby. And I think it should stay like that. When they get better in those lobbies, they get promoted and if they can't hack it, they go back into the bot lobbies.
Sbmm needs a major revamp to allow the average player to enjoy their experience. I have a k/d just over 1.3 (when i started it was 0.7), and I'm getting into games with apex predators and 3 stacks regularly. I cannot get better at this game as a gold 1 to plat 4 player playing in those lobbies. I'm a bot now. I sometimes get lucky and get a lobby with guys who are my skill level and get a 5 to 8 kill game. I play ranked and in bronze and silver i dominate, gold i do well, and in plat i struggle. Why am I in those upper lobbies?
My solution, I'd like EA/Respawn to see this....
10% of lobbies are bot lobbies with k/d's under 0.4 and win percentages under 2%..and new players up to level 10 stay in these lobbies until the code figures out where they should be placed. If they suck, those players will always be protected. If you get better in those lobbies you get promoted. If you can't hack it and your k/d drops and win rate, you go down to the bot lobbies again.
Remaining 90% of pub lobbies have the remainder of players. (bronze 5%, silver 40%, Gold 35%, Plat 16, diamond 2%, pred 0.2%).
So the lobby will be comprised of those skill levels. Basically..it will take you longer to find a game if you're terrible and bronze level players k/d's around 0.4 to 0.6 with low win rate, and it will take longer for you to find a game if you're good, plat and above (k/d's over 1.7 and win rates above 8% say).
This will fix apex pubs imo. Means the lobby will have 12 players (20% of 60 players is 12) with k/d's above the normal tier. You may run into 3 stacks still (it'll be much harder for them to find a lobby together), but the majority of teams will be even. There is still sbmm, but it isn't alienating the average player anymore.
This needed to happen yesterday for pubs.