I agree with all your reasoning for this, but why "switch"? I mean, if a player can switch actions freely, then what's the point of declaring them in the first place? It sounds like maybe you'd be fine with only certain actions being declared.
You'd still need to make some type of declaration in order to determine how the group will proceed when initiating combat. How actions are handled from that point (or even throughout the round) may vary widely depending on which type of initiative is used: Individual, Group or Phased (and which type of Phased initiative, which may also vary significantly). So it's hard to establish one method that will apply to all initiative styles, but generally speaking once you set a course of actions that decision may compromise your position even if "switching" actions after the fact is allowed.
If you opted to move to engage a target in melee several feet away, for example, but by the time you got there someone else in your group who was already engaged in melee at the start of the round defeated the specific opponent you intended to attack I might allow you to attack an adjacent opponent instead, since you're already there. But I would not allow you to attack a distant opponent that required you to move again, since you already moved that round, so your position would already be compromised by that point. You may either take another action from that spot, or lose your round if no other action is possible at the time.
Interesting. I see it pretty much entirely the opposite way around - Alice: "Attack orc #1". Bob: "Attack orc #1". Charlie: "Attack orc #1". DM: "Orc #1 falls over dead". Evan: "Attack orc #2"... with everyone distributing their attacks perfectly optimally because their characters are able to instantaneously respond to any change in the circumstances of the fight feels far more gamey to me than to require players to take the risk that some attacks may be wasted on beating an already-bloody corpse if everyone focus-fires a single foe.
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Sure, quantizing time in a series of rounds does introduce differences from the real world, but, even with eyes, it takes time to adapt to those evolving circumstances. If you're ferociously raining blows down on orc #1 and your buddy takes its head off, you can't just redirect your next swing to orc #2 and continue raining the same ferocity upon the new target. It interrupts your rhythm, you have to check and redirect your swing, you need a moment to adapt to the evolving circumstances. Which, in game terms, is "new action in the next round", because rounds are the smallest division of time available. (Obviously, this makes more sense with GURPS 1-second rounds or most current games' 5- or 6-second rounds. In the 1-minute rounds of early D&D editions, there would be plenty of time to change your action, probably multiple times.)
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Yes, and you've made your series of thrusts and parries and been looking for an opening to attack your original target. When you switch targets, you need to start that whole process over again (*cough*next round*cough*), you can't just say "orc #1 left his right knee open, but his head got lopped off, so I'll stab orc #2 in the right knee" and expect the second orc to have left the same opening.
Yeah, but that "risk" taking factor assumes that battle decisions are something that must be entirely decided head of time and that once that course of action is taken no adjustments can be made on the way there till the following round, which would take place several seconds later (at least in almost all systems other than GURPS, AFAIK). But if that was the case I would crash my car every single time I'm trying to speed pass a slow line and someone randomly shifted lanes in front of me, rather than me being able to break and perhaps shift lanes as well or do other adjustments instead of instantly crashing my car cuz I have to wait till the next imaginary round to adjust my course while driving. And that may well turn out to be the case, since many people crash exactly that way, but I never have, because reacting to other people's bad driving and adjusting my course midway is
also a possibility (albeit, at a risk, which may require an ability roll in terms of the game rules). And crashing a car as a result of someone throwing their car at you is an infinitely a more likely scenario that me beating a bloody corpse like a retard cuz I have to wait for the next round to tell that its already dead.
Taking a swing at someone beside you is not that difficult, and if anything the fact that you're not directly engaged yet may grant you an opening since they're not properly positioned to meet your attacks effectively. Its only once you've both had the chance to properly position yourselves and raise your guards that the whole song and dance of thrust and parries clashing against steel really starts, but a random attack may still find its way in that time and catch you unprepared. Granted, by the same token an adjacent enemy may also take the chance to take a swing at you (as they would in my games) if it's one of your friends who takes the fall. And the case could also be made that since neither of you is properly positioned to face each other that your attacks should take a penalty, if allowed (which is something I have seen in games that explicitly allow you to switch actions mid-round in the rules). But generally speaking adapting to an enemy falling while others are still near by takes only a fraction of a second, which is more than enough time in most games, which use 6 second rounds (which is enough time to kill several people in a real life encounter, and many, many more if we're going by old D&D's absurdly long 1 minute rounds).