It is not complex, it is just more complex than not having bonus actions at all. Mearls' point is that there is fundamental need for bonus actions for the game to work fine, they are essentially redundant, and eliminating them would decrease the complexity a little bit.
But notice how their existence creates a cascade of small but yet additive complexity increases:
- they could have just stopped with bonus actions being in addition to everything else on your turn
- immediately there's the problem: what if I have 2 special abilities that use bonus actions? hence the need to specific "maximum one per turn"
- then next issue is: what if the bonus action is a spell and I also cast a spell as my main action? so you need another patch i.e. require that the other spell be a cantrip to avoid it being too much in one turn
- then you have other unforseen consequences and corner cases such as when the PC has already cast a spell that was not a cantrip, and can't cast another bonus action spell, or when using Action Surge or other abilities that grant more actions, and you have characters with special abilities for bonus actions...
So they are not "complex" for the average seasoned RPGer who learned to play with the pointless redundancy of certain older editions, but these complication can certainly put off a lot of beginners and casual gamers, many of which can't stand the nonsense and the triviality of rules minutia.
He probably just answered on the top of his head, but this is not as good as it could be. For example it's quite unfair to Arcane Tricksters because with this change they can only either cast a spell or benefit from the improved mobility granted by Cunning Action (which is the whole point behind its design).
I think you highlight why having bonus actions as a defined game mechanic is so useful. The bonus action interaction with other game mechanics does take a few column inches of space, but the alternative is that the interactions for each bonus action are built into the bonus action text themselves. This multiplies out the same interactions over and over and over again, especially with bonus action spells.
The mere existence of the bonus action mechanic having only 1 per turn cleans up so much of this and saves column inches because it consolidates the "half actions" and their interactions to only a few spaces. It also makes the half actions loosely coupled instead of being tightly coupled to the other actions they work with.
Some spells can be rewritten. The smite spells could reasonably be integrated with just the attack action. Some spells like
divine word could be actions on their own. Other spells, like
expeditious retreat, could be written to allow you to take the Dash action along with another action each turn.
Other spells might not work as well, and I think
green flame blade is an example of what a world without bonus actions would look like. The language has been confusing for some, and it could have been easily changed to a bonus action with a duration of 1 minute that took effect the next time you hit a creature with a melee weapon attack. The adjacent creature then needs a dex save to avoid taking the fire damage. This would work with two weapon fighting and extra attack to have additional opportunities to use it instead of turning your entire turn into an all or nothing affair. Only the fire damage becomes all or nothing on a save.
Still other spells and half-actions turn into a case of, "now what?" in terms of what they allow. I think
spiritual weapon is a good example of where things can get out of hand. It is too weak to be a full action. So now does the extra text say that you can just use any other action? Does it restrict what the other action can be? If not, a creature could use
spiritual weapon with any number of other actions that allowed any other action to be used, or its power is increased to be on par with a full action spell and controlling it takes your action on subsequent turns. Other bonus actions like Cunning Action should not be restricted in what the main action should be. So then a creature could cast
spiritual weapon, Dash, take any other number of half actions that shouldn't be restricted in what the other action should be, and finally finish with the real action, or we can just have bonus actions restrict this to 1 half action a turn.
There are any number of ways to deal with this interaction of actions and "half-actions", but I consider the bonus action mechanic to be elegant in its means to consolidate rules and column inches rather than a kludge. I use bonus actions for several of the abilities in the 5e version of
War of the Burning Sky, and it works really well to allow a bonus perk while keeping the flexibility of the main thing the player wants to do.