Where do we draw the line where it becomes a F2P game with a mandatory subscription?

Sure, some people would pay it. But the current game design just doesn't have the same addictive gamble appeal as the big F2P money milking games do and a subscription on top would kill it. If SE went too far down that road, they'd simply cut the subscription and go all in on F2P milking because it would lure in more players than a monetized subscription game. But it would involve redesigning the game towards a different audience. It's not a good path to go down.