"Agree" is not exactly accurate. FIFA tells them. And until 2022 it probably won't be worth it to go against the grain.
I think you are also overestimating Fifa getting clubs to agree to let players go. Yes they have reserved fifa international dates, which require or compel players to be made available, but if the national team wants you to go and you want to go, you are probably going. Man city lost yaya toure, their best player, for three weeks in the middle of the season to the Africa Cup of Nations, which did not occur during Fifa dates
The tournament would be in the summer and clubs would have a hard time preventing players from playing for their country.