Bonus actions in 5E ARE hacky and too decoupled from the fictional world. There's a lot of places in 5E where you get to do something "as a bonus action" but no consideration is given to what you are actually DOING. Bardic Inspiration is a poster child for this kind of ugliness. If bonus actions didn't exist, presumably the bard would have some kind of ability like "When you cast a spell through song, you can weave your words in such a way that [the ally gets bardic inspiration]." That would make it clear that (1) yes, Bardic Inspiration is compatible with spells that have verbal components; (2) you can't inspire others while gagged (even though you can cast spells while gagged, e.g. Hypnotic Pattern, because it has no Verbal components); (3) it would make it clear you can't inspire others with song while you are busy drinking a potion (Action: drink a potion) because your mouth is busy. The ability could be written in such a way as to be compatible or not compatible with other actions like hiding, attacking, or extra object manipulation as desired. The designer would be encouraged to consider in advance what the ability is really doing from a fictional perspective instead of just lazily slapping gamist "bonus action" jargon on the ability and ignoring the fiction, leaving it all for the DM to fix during gameplay.
Also, bonus actions have weird interactions with other rules like Readying actions: technically, you cannot ready Bardic Inspiration or a Misty Step, although it isn't at all clear from a fictional perspective why that should be the case.
I agree with Mearls that bonus actions in 5E are poorly designed. I might not agree with him on a given proposed solution, but he's right about the problem. They encourage lazy and poor design.