I would not accept an argument which produced an answer I knew was wrong regardless of whether or not I could find the flaw in it.
Perhaps, but this is not a helpful position in a debate.
There's a lovely proof I saw once that all angles are equal to the right angle. It took hours to find a point where the proof contained an error, and it was a really subtle error, but I could reject the conclusion immediately because it couldn't possibly be correct.
But there is a triangle where all three of its internal angles are each right angles. Clue: its non-Euclidian.
Personally, I think the flaw is in the assumption that the wizard's ability to choose targets and other people's ability to interrupt the spell are both contingent on the passage of time, and thus that if there's enough time for one there must be enough time for the other. I think it's quite possible that the wizard's ability to choose targets is a result of a general principle of how the rules are intended to work and the question of how much time that takes is being ignored because this is fundamentally a set of game rules, not a simulation of physics.
Its not the ability to choose targets that is the problem. I have no problem with a wizard simultaneously and instantaneously choosing four different targets, one for each beam.
The problem is that time
must pass if he is to observe the effect the damage of one beam has, and only target the next beam
after that observation.
Observing the result of you spell is not part of the casting process; its a mundane action that can be done for any observer, and takes the same amount of time (however long that is) for any observer, including the caster.
There is no suggestion that the caster of a spell can see into the future to divine the effects of beams he hasn't shot yet.
The game is not a simulation of physics, true, but is a simulation of an artificial reality of a game world. Things still have to make sense in that world. If not, it ceases to be an RPG and just becomes an arbitrary board game. You could certainly make up a rule that says, 'Wearing the colour purple gives +2 to your AC versus attacks from your left flank', with the 'justification' that 'the rules say so, because I just made up the rule that says so!'. But would that rule be satisfactory? If nonsensical rules abound, doesn't that take away from your immersion in the game?
Why have rules which don't make sense when you could just as easily have rules which do make sense? Why rule that time works differently for the caster of a spell when you can just as easily rule that time works the same for all?