AlphaElwedritsch You only need a single app that you are dependent on. Be it a bank, insurance company or government app. It doesn't matter, just a single one that doesn't work.
And then the whole GOS is gone. Or any other (security) custom ROM. No matter which one.
GOS can't do anything about it either. The problem lies at the core.
Then you just have to switch to stock. No matter what the cost.
Or carry a second device, which is something some users already do. We know this because of posts from users about carrying around a mobile hotspot plus a GrapheneOS phone.
Obviously that would be a major inconvenience, and it is possible that many current GrapheneOS users might switch back to the stock OS. But each of those users would be choosing to do that instead of carrying a second device.