For most tournaments, the commitment for players is five rounds: four in competition, plus a pro-am day beforehand that they are obliged to attend. Interestingly, it seems that more tournaments are having two pros tag-team for nine holes each in the pro-ams now, but that's still a fifth day the players have to be away.
The pro-am often helps fund the tournament, so that's not going away. And yes, many players would rather have three days of competition instead of four, if the money was relatively the same.
LIV is hardly the only circuit to play 54-hole competitions. I believe that most college tournaments are 54 holes, until they get to the NCAAs. A lot of LPGA events used to be 54 holes.
Does 54 holes identify the best player as well as 72 holes? Probably not. But is that really what tournament golf is about? Arguably there would be more pressure on the better players to stay focused in a 54-hole event, instead of coasting on the assumption that a lot of the field is going to fall away eventually. Bobby Jones did say that the hardest event to win was the Amateur Championship -- a bunch of 18-hole matches where you are always in danger of running into a guy playing over his head.
We do nine-hole matches for the Renaissance Cup precisely because it increases the odds of an upset . . . but winning five matches in a row does take some fortitude.