And yet, D&D has playtested simultaneous targetting for magic missile for over 40 years and there have been no problems mechanically or with complexity.
Scorching ray has had the 'simultaneous' wording since the last millennium (that's a thousand years, people!), and the game didn't break.
The complexity is equal, with regard to simultaneous/consecutive targetting, so choosing one over the other on grounds of simplicity of use is a wash.
If something makes sense it tends to have few if any problems. If something doesn't make sense problems appear.
Having scorching ray and eldritch blast working in 5E just like scorching ray always did before, simultaneous targetting and release, is no more complex to adjudicate than consecutive, keeps 'instantaneous' meaning what it says and means the spell has come and gone before it can be targetted with a readied dispel, and matches the description of the spell effect (beams of energy streak towards the targets). You also don't have to wonder if the spell effect, or some of it, appears before you've even finished casting the spell, or wonder if the Cast A Spell action actually allows you to cast four cantrips with the same action.
Having these spells work consecutively, with enough time between beams/rays to see what each does before deciding how to use the next, brings problems without making it more simple to play. How can an 'instantaneous' spell give you time to think between parts of it? How can it give the caster time to do that while not giving a readied dispel the same time? Or, if it does, we now have an instantaneous spell which does hang around long enough so it can be dispelled! Why is a spell that functions like this described as 'instantaneous' when they could have the spell work the way they want by giving it a duration of '1 action' or '1 turn' or 'special: see text'? Why is the spell description ignored (beams streak toward targets) and replaced by non-existent wording which changes the spell effect from 'beams streak toward targets' to 'the caster is empowered to create shoot four beams during this action, one at a time'?
There are some rules which are especially unrealistic in 5E particularly, but we can accept them because they make the game much easier to play. My favourite example is the 5E version of non-lethal/subdual damage, or knocking someone out instead of killing them. It is absurd that you can stab someone for actual damage, find out that this kills them, then retro-actively state that your stab was not a stab but a knockout blow instead, changing the past. Absurd, but we accept it. Why? Because we've played the previous systems and found them more complex than it's worth, so we just roll with it. It doesn't create any other problems.
But ruling consecutive beams over simultaneous creates a whole host of practical problems beyond the mere absurdity; it doesn't actually make the game simpler!
In play, the DM says that 10 baddies are in range; name the target for each of your four beams. You say that you'll aim two at the BBEG and one at each of his two main henchmen. Cool, roll your attacks.
This is not more difficult than picking a target, rolling to hit, see the result, aim at same guy if he's still up or a different guy if the first fell down, four times.
There is no reason to use consecutive beams beyond "I don't want to waste a beam". Well, suck it up, big guy! You don't hear the wizard whining about wasting magic missiles or paladins wasting a crit smite on a BBEG who only had one hit point left! Okay, you do, but you don't change the way the rules work to mollify them, creating rules carnage as a consequence!
Simultaneous targeting isn't any more complicated than sequential targeting, it is having two different targeting methods in the same game that would make it complicated.
