By the rules, according to the sage advice compendium actions and bonus actions are not interchangeable.
I can completely see why you shouldn't be allowed to perform an action as a bonus action, because the action economy is a big part of balancing.
However I do not understand why you shouldn't be allowed to perform a bonus action as an action. For example the description for spells with a casting time of bonus action in the PHB 202 states:
A spell cast with a bonus action is especially swift
So if the spell is just faster, why can't I use my action to cast it and then stare wholes in the air with the rest of the time?
To me it seems reasonable to allow players to use abilities that need a bonus action as their action. So I am considering house ruling this, because it comes up rarely but if it does it's annoying for the player that he can't.
In our special case the fighter/barbarian multiclass just got hit quite hard and he wanted to use his action and bonus action to rage and second wind for the damage resistance and heal. I didn't allow him because I knew that by RAW it's not allowed. But thinking about it I don't really see a reason to forbid him.
So my question is: Are there any balance implications if I allow my players to use their action to make bonus actions?
If important, we are only playing with official and tested content, namely PHB, DMG, MM, VGtM, SCAG and XGE.