People do manage because the end-game content difficulty in XIV is subpar. Also the way instances are made currently makes ready check not really a must have.
Right now, only challenges are : 17-18min speedruns / Garuda.
Speedruns: No time for ready check during the run, have to move fast. Also only one mini-boss and one boss, so you keep on moving constantly.
Garuda: Short battle as well, only "one" pull, then you remain engaged and focused the whole fight. Ready check would only be useful before entering, but since once you enter, Garuda doesn't attack, it's not a big issue, because the 30min time limit is barely a restriction.
Then, party are only 8 players, 7 minus the leader. Even if not convenient, it still possible to check manually if players are there or not as long as the party is small.
Using that argument of yours "how do people manage without this idea", we could simply remove everything that was implemented in the game during the past years. Who need improved market wards! We could manage without on XIV 1.0! Who needs mounts!? Who needs improved looking for repair? Who needs... Let's go back to XIV 2010 shall we.
More importantly, who needs players that refuse to comply to leader's orders in a raid? No one. So again, don't use the feature, enjoy being removed from groups, no one will ever miss you. You are expandable.