The mouse swing is the best innovation for golf games in the last 20 years.
The players leaving BECAUSE of the mouse swing certainly aren't the problem numberwise. The slow development is the problem. The game is unfinished and it lacks important content.
I'm sure the reason for players leaving is a combination of all the above but I must strongly disagree with your statement regarding the mouse swing. Perhaps if I was coming from the point of view of a 3 click player who'd never experienced a decent real time mouse swing I could agree. First of all, it's not even a real time mouse swing, for which there is zero excuse. I've personally coded a real time swing in both Unity via C# & UE4 via blueprints with intermediate programming skills at best. It is not that difficult and has been part of many game titles dating back to Front Page Sports in the 90's.
Secondly, the fundamental effect of tempo affecting directional draw/fade bias makes no sense. I made dozens of posts on the old beta boards regarding this key topic, requesting at the very minimum a simple option to turn the tempo effect on/off, and all were essentially avoided by the devs. If they had implemented the tempo calculation to impact the quality of the strike and overall distance potential, now that would be logical. The forced tempo requirement actually reduces any sense of feel in the golf swing. Instead, as in a real golf swing, they should have accounted for an acceleration factor in the downswing. Combined with the existing off-axis swing plane code (which is very well done) and modified tempo calculation impact, you would have an outstanding mouse swing. IMO, the "real time" controller swing at Tour Pro skill level is by far the best choice in JNPG simply because it does NOT include a tempo component.
BTW - In no way am I asking for the swing to be made easier but conversely, all suggestions made were to add needed depth to the overall swing code. PGA2000 had possibly the best real time mouse swing to date, that also included a wonderfully implemented tempo factor, which the game code would tailor to each user by learning their personal swing motion. As another user mentioned, no need to re-invent the wheel just to be different.