Guess I have to play the devil's advocate.The reason for playing a game does not rely on completion but satisfaction.The latter gradually drops as time goes.The only way to maintain is having more powerful character.Now,since a new character is guaranteed every 2 weeks and you can have at most 20 units active.Hence the "minimum lifespan" of the game is 40 weeks.
Given the fact that these event units are slightly weaker than most 5* units but stronger than everything you can have with the free gacha, had you been utterly unlucky, it is possible to halt entirely once you have 20 event units.

It is not too bold to say that it is gameover once you have 20 6*, and for the more hardcore players,2000 amps of each type, and for the whales, maxed skill and equip slot.My team formation has not changed much after a week I started and now,except a few more event units.Once that is exhausted, there is no more point in playing.(and yes,I acknowledge there would be second evolution and other features but if that serves as an argument,one may as well play the DMM version)

It seems to be missing out a lot if one quit, but the truth is the inherent limitation has made the game bound to be mundane given enough time. The factor of luck is and may be the only decisive factor in the game when it literally all depends on random note and gacha.

TL;DR
Be informed about the unavoidable fate of the game and decide if you are going down the road.