It's not like you really need 5* girls to complete the game.

You can pretty much clear all stages with a full team of 3* as long they are properly equiped and upgraded. Not to mention that you can get 4* from the free gatcha seeds. And some 4* are really strong, such as Cepha Lanthera and Lilac. Also it's not like you will get much improvement from the 50 gems gatcha. Since the begin of the game, I got just a single 5* from it (and got it after clearing all the main stages) and never had any dificult to progress the game.

And last, but not least, you get a free 5* every two weeks from the events unless you skip them, wich you never should, since the events are also the main source of free flower gems.

So your reasoning of leaving the game 'because you can't upgrade your team" is pretty much nonsense.