I guess they count on that eventually players will adapt to expanded format across competitions such as UCL and World Cup. The first few seasons should be nasty because they aren't used to playing a game every 3 days but a few years more and they'll probably get along with it. Idk about how exhausting it is though, probably really much. If they all agree it's unadaptable then this shit needs to be fixed. Like Rodri said, if you want quality matches then it's not a good idea to have them every 3 days. Players are tired and hence less spectacle and less action on the pitch.
Well for this to happen then you need to relax the salary caps and allow for bigger squads. So either way you are reducing quality.