While an action economy is in a sense implicit in EVERY game, IMO Mearls might be saying that he didn't want this to be so prominent as to feel necessary to think in terms of it when playing the game.
Remember that 5e had a primary design goal of allowing as many playstyles as possible. Action economy micromanagement makes for a specific playstyle, a valid one, but something they likely didn't want to become mandatory.
IMHO they mostly succeeded but "mildly failed" in a couple of things:
- not describing explicitly enough the simultaneity that is behind 5e actions, particularly with regards to bonus actions
- adding the dissonantly over-precised object interaction rule
Since a bonus action is by definition quicker than an action, you'd think it would be possible.
This is example of WotC own failure at explaining things clearly. Bonus actions are not faster by definition, but most people are lead to assume they are because they are a more scarce resource and because some bonus actions are attacks and thus can be compared directly with using an action with Extra Attacks, resulting in something less valuable.
The truth is that bonus actions are merely a rules artifact. You could theoretically have a bonus action that granted a bigger outcome than your action.
And this leads back to the idea of the simultaneity of all your actions in a turn (where their serial resolution is more for convenience than true meaning). If bonus actions were presented as improving your actions rather than adding to them, maybe there would be a more natural flow to combat. I think this is pretty much what goes in Mike's mind when saying the game would have been better without bonus actions.
