The way I see it, with crossplay coming there is absolutely not one good legitimate reason left to not have solo mode implemented other than the dev's ego.
"It would split the playerbase too much !"
Crossplay with the absurd console playerbase just makes that statement null and void.
"It would just be one pred dominating the game !"
Comparing to pubs and having 3 of them, I fail to see how this has ever qualified as valid argument, one pred is still easier to take down than 3.
"Some legend wouldn't be played because bad/useless passive! It would make the gamemode repetitive"
That is sort of a fair point, until you realize this is personal opinion, if you don't enjoy the meta in one mode, don't play that mode, what do you want me to say ? Rev wasn't meta on release, didn't prevent people from playing him. At the end of the day that argument is just whining about people enjoying one toy you don't enjoy.
"If you don't like team based games don't play Apex"
Well, I do enjoy squading, what I don't enjoy is how garbage an experience it is when you don't have friends playing apex, you get leavers, griefers, people way below your and the entire lobby's skill level, you get loot goblins and if on the summary screen adding up both kills and damage of my teammates and multiplying it by 5, it still isn't exceeding mine there is something wrong with matchmaking.
"Just make friends"
I'm at an age ( actually has nothing to do with age ) where I really don't wanna bother looking for a group of strangers every time I wanna play Apex. Also, I decided on a lone wolf character setting 10 years ago and I'm staying true to it dammit !
"If you want Solo just boot the game in casual and wait for both TTVs to leave"
Let's face it, the current soloQ experience is just a hot pile of steaming shit, just throw us a bone and humor the community if you don't plan on making it better Respawn.
Edit 1 : For the people who are against the idea of a solo mode in apex, why ? What's in it for you that you don't like it ? It won't be the only mode left, like with duo, it won't suddenly kill the game, so why gatekeep it ? I wanna be mean and say because you really, really enjoy those games where you get carried, but I'm sure reddit isn't that shallow.