Monsters don't need anything to make up for it. Unless you as the DM are constantly moving monsters out of range without using Disengage, the difference between the players and monsters shouldn't really affect the balance between them. If it's only occasional, it won't make a difference. If multiple PCs are getting OAs every round, then it would shift in the PCs favor.
I think it would likely have one of two outcomes, either PCs don't take advantage of it and just stay in contact with the enemy. Or the PCs are constantly doing hit and runs, in which case the enemies are free to move around at will and hit whoever they want. Which would likely make the PCs revert to staying in contact with the enemy in order to take advantage of OAs and prevent enemies from getting near the weaker party members.
It would definitely change the effectiveness of rogues' cunning action, swashbucklers, mobile feat, monk's step of wind. If you already have a part with those things, I'd be hesitant to change or at least try to find ways to give them other options to make up for it. If you are starting a new game with this option, I would make sure players know that and the effect it would have.
Dual wielding rogues, for instance, would now become more common. After hitting and doing sneak attack on their first attack, there is no longer a choice of a second hit for a little more damage or using a bonus action to disengage to avoid a hit. They can now do both.
It's not something I would do or want to play with, but if it makes it more fun for your group, then go for it. But definitely make sure that it isn't negating a player's build/style. If one of your players made their character to optimize those hit and run tactics, doing this will make them feel less special and could make the game no longer fun for them.