Thank you for a reasoned response.
I must point out that, even if JC rules that the beams are sequential, this doesn't mean that the warlock has time to observe the results of the first bean before shooting the next.
Remember, if he does have that time, then so does the dispeller. The rules that JC wrote are that instantaneous spell cannot be dispelled,
because the magic exists only for an instant. If JC now rules that 'instantaneous' is long enough for the warlock to observe results and change targets based on that observation, then he has also ruled that the magic
does exist long enough to be targeted by a dispel! This would mean that JC's tweet disagrees with JC's rules! See my problem here?
Of course, this may be a result of a limit of 140 characters....
As I pointed out,
if JC is actually ruling that the beams are sufficiently separated that there is enough time to see the result of a beam and react to it, then JC is indeed disagreeing with JC!
You
could argue, but not successfully.
The entry of 'instantaneous' tells us that these spells cannot be dispelled, because the
magic exists only for an instant. The 'effect' of a spell
is the 'magic' of a spell! The 'magic', the 'effect', can certainly have non-magical consequences (like damage), and these consequences cannot be dispelled by
dispel magic because they are not even 'magic'!
The 'magic', the 'effect' of
eldritch blast is crackling beams of
damaging energy, streaking toward the targets.
That is the 'magic', the 'effect', and the
only reason that they cannot be dispelled is that the 'magic' comes and goes in an instant, meaning it is no longer there to target by a readied dispel.
If you, or even JC himself,
change 'instant' into a period of time within which you can do other things, then the very reason they 'cannot be dispelled' falls away!
So, what
are the 'effects', the 'magic' of this spell? Is it the damage? If it is, then the damage, as the ongoing effect, could very much be dispelled! But the damage isn't the 'effect'; 'damaging beams of energy' are the effect.
In which case, they
can be dispelled and therefore no longer fit the description for 'instantaneous' spells.