To expand a bit...
When Mike is talking about "planning your turn" I see that underlying idea that your turn is a single thing.
One step towards that was splitting up the move: in 3e you had to move either before or after your action, while in 5e you can split your movement. This is conceptually consistent with the fact that maybe you are moving with your legs (or wings) but you are performing your action with your arms, so action & move happen effectively at once.
Even though some things inevitably need to be resolved in order, the idea of simultaneity can be seen also in bonus actions: "Healing Word" could be seen as a bonus action because while you are attacking (or whatever you are using your action for) you can also speak a magic word that heals an ally. The Fighter is two-weapon fighting because she is actually using two hands at once. If you have a free hand, you can gulp that potion while you're still using your other hand for an action. The Barbarian rages as soon as he attacks, and so on.
You can design an edition which still has all those additional things to do, without having "bonus actions" as a game construct. It may be however, that you'll have to spend more words for those descriptions, while codifying "bonus actions" simplifies the job for the game designers who now have a category to rely on.
The drawback of the more traditional sequential actions economy, is that sooner or later it prompts players to think of their turn as a set of boxes to fill: "what am I going to do with my bonus action this turn?" isn't even a question if you see everything as simultaneous, but then it causes some players to be frustrated when they realize they don't have enough bonus action abilities, and may even think they made some wrong character build: "why does she get to use a bonus action every turn and I have no bonus actions? my class sucks!". It also spawns unreasonable and unnecessary discussions like "how many of the 6 seconds round is spent by an action vs a bonus action, and therefore can I swap my action with another bonus action (or two)?".