An owner can cut a player's pay any time they (or the GM on their behalf) negotiate a contract with a player. If they don't want to be locked into a long contract with a player, don't sign a long contract. You may lose the player, but that is the market. What the league is effective asking to do is to not honor every contract which still has time left on it. I feel that is rather extreme.
You seem to be under the impression that the league has the power to force players to play for whatever salary the league deems fair, and the players have no other realistic option as to where to play, so they should just take it. The players meanwhile seem to collectively feel they have a very valuable skill set which is in (no small) part is due to continued growth and popularity of the sport. A skill set which, while there aren't many other employment options, is very hard, if not impossible, to replace.
The league absolutely has the power to make this their final offer. Your assertion if this is reasonable I think depends on if they are content in not having a product to sell. They apparently are. I don't think that is reasonable in the slightest.