Technically most of the game is optional. Even choosing to play is optional. I'm not sure why this justifies putting a cash price on things. Basically we play to enjoy lots of different optional things and aspects of the game, we play because we enjoy glamour and collecting mounts and pets and making our own characters special to us and enjoying the story and different features of FF. All optional.

What the cash shop does it take countless items that we'd enjoy and stick them behind an additional paywall. There's a limit to how many things you can restrict before the quality of the base game is affected. I mean, imagine how much more enjoyable the game would be if 75% of the cashshop was available ingame? That's a significant number of amazing looking mounts, pets, glamour, emotes and other things to chase, farm for and acquire. Imagine too how much more enjoyable the game was if we all had, say 4 retainers default and double saddlebag space free, it would make the game feel so much better.

The cash shop has passed that limit. It's absolutely massive now and they keep stuffing more and more into it. Which means the basic game (which costs money too) suffers for it.