I agree that the game should allot us to convert our main action into a bonus.
If my ranger would rather take another potion instead of attacking the enemy twice, he should be able to.
If my warlock would rather crouch and hide instead of casting a super complicated spell, she should be able to do it.
I would never say no to a player asking to replace the main action for another potion or another simple action that could be done with a bonus action.