While I would love nothing more than a solo mode I look at it in a different way than most. Having been around during solo mode and playing it people often forget the horrible outcome it had on the game.
For one the game mode had teamers as mentioned. We are not talking about 1-2 people teaming we are talking about 4-7 people all teaming on a single building. The queue times are also heavily affected. While some say oh this game has no issues, people need to understand that this is apex legends. Totally different game and play style from any other game out there. We have abilities which created a very unbalanced system for a lot of legends having their legend of choice be utterly useless in a solo mode.
Then comes the biggest issue of all. I often see people complain about sbmm in apex. You are paired against preds and masters and some are only plat or below skill set. If this happens in pubs I often wonder how they would feel about half their solo lobbies being full of these players who will just wreck them. Then comes the issue of players who begged for duo game mode. Once the duo was released and no fill was added it was not enough still they complained. They were upset about the sbmm and how they stand no chance against these players. Well solo would be even worse. There are only so many players who fit specific stats they look for with sbmm. So if your gold + expect to be placed with some high-end players.
Then comes the biggest issue of all. On NA servers master/pred queues can take up to 5-30 minutes for a single match to finally be found. This is bad for the ranked scene as is. Instead you will have preds/masters who just want to run around farming lesser-skilled players and feel good about it. People may assume all they wish on how this game mode could and would work flawlessly but honestly it holds to many concerns to be thought most likely in their minds right now.
The game mode itself had a very negative impact on apex and this is the reason it has not returned or a permanent game mode. Even if people think they know whats best and they know it all and think its a viable thing to add. Those of us who was around during solo game mode when it first launched can honestly say what issues they seen with that mode and how impactful it truly was.