Problem is, D&D does take combat turns no matter which system you choose: the default or the Mearlsian one. Furthermore, D&D combat already assumes that you are constantly doing what you can to survive a combat round, even if it's not your turn. You are "avoiding" hits when the foe fails to hit, and you are looking for your next opportunity to make a hit.
You don't have to stick to those two. Historically in AD&D, there were only rounds, not turns--all player decisions were made concurrently, up front, at the beginning of the round (see Rath & Delsenora example in the 2nd edition PHB), not on separate "turns." Turns are a toxic concept and function primarily to serialize player interactions--to prevent players from taking up the DM's attention when it isn't their "turn". (And then some DMs act surprised that players tune out in between their turns! If you exclude players from participation in 75% of the game, don't be surprised when they pay attention only 25% of the time!) Just throw them out and go back to rounds: everybody declares their action for the round, then everybody resolves their actions, rolling initiative if necessary to resolve conflicts.
We see certain complaints in this thread about being unable to do certain things on your "turn" because you declared something else at the beginning of the round. That's a sign of an impedance mismatch. You shouldn't take input from players both at the beginning of a round and on a "turn" within a round, or you wind up with frustrating resolutions like "you said you were going to attack the orc, but you didn't say you were going to move, so now that he's stepped back 10' it is now your turn but you can't move since you didn't roll a move die, so you are forced to declare '
I do nothing.'" That's insulting and unnecessary--it just rubs the player's face in the fact that his decisions are being artificially segmented by the initiative system you're using. Instead, just let him declare the action once ("I attack the orc") and when it comes time to resolve the action, resolve it, using dice if necessary. ("You advance ten feet and [player rolls a 7, total = 12] strike at the orc, but misjudge the distance and your attack falls short by an inch.")
If two players both declare that they're attacking the same orc, and one of them rolls a killing blow while the other one misses, then fine, they kill the orc. If they both roll killing blows, again, fine. You can have an initiative contest if your players want to see who gets bragging rights on the killing, but regardless, they both accomplished their goal for that round: that orc is dead.
It's simpler and better than PHB cyclic initiative.