Well, recently we've seen how netmarbel is making it's best to milk Grand cross.
For example, they added 6th star for super awakening.
Before that, they added artwork cards.
and they're going on at this way.
One day you may find yourself forced to quit the game whatsoever the reason it could be.
It is recommended for such games to play safely I mean not pay much or even not too.
Because, once you walk away when it gets too dark you won't regret that much. I'm not saying this to argue against the game or buying gems not at all.
developers can make better updates for the game in which it can be more developed, such as, walking in open world and hunting monsters instead of loading screen.
however they are adding new stuff to make the game obsolete and this will end up making players quit unfortunately. Therefore it is better for the developers to plan something better.