You don't expect what you cannot choose to do... The only thing you can eventually do is looking at the initiative order and "try to imagine how things are going to go". Overwatch doesn't allow me to target the ennemy that's about to fire a missile and it's perfectly fine.
Larian doesn't absolutely have to use every words of the PHB to implement reactions. It could be different and still be valuable tactical option to use in BG3... just as overwatch is a valuable tactical option in Xcom even if the rules doesn't allow you a full control over "when and who".
I'm not comparing overwatch and counterspell at all, I'm just giving exemples. Tactical turn based games doesn't absolutely need a faithfull DnD 5e reaction system to be fun and to allow our characters to react during the ennemies turns. Especially if it means "chopping up" the turn based combats even more.
I totally agree with you that BG3 doesn't need to be absolutely faithful to D&D 5e reaction system. If I felt that way, I wouldn't be considering presets at all. If you want a full and true absolute D&D 5e implementation of reactions, the prompt/popup system is the only way to go. It's the equivalent of a DM saying, "This goblin just ran past you. Do you want to do AOO?" to new players who have absolutely no idea that they should interrupt the DM and say, "Hey! That guy ran past me. I want to use my reaction to do AOO."
So, let me rephrase what I'm saying. I'm good with presets as long as presets are not as simple as "On/Off" or even just as simple as "Target These Specific Enemies." What I'm saying is, "If Larian is going to use presets for reactions for BG3, those presets need to be at least specific enough that players will not get pissed off because the automatic preset system screwed them over. AND... some reactions may still need a prompt to make that work, but the preset would help to cut down on the frequency of the prompts. This doesn't have to be an ALL PROMPTS or ALL PRESETS system. It could be a blend of both."
Let's use a different example since people seem to be hung up on me using Uncanny Dodge a lot or Counterspell. AOO. Simple presets would take care of most of them. Preset options are: All Enemies/ Selected Targets/ Off. For this one, you either have it trigger like Overwatch in Xcom where the first person to exit melee combat without using Disengage triggers your AOO or you have selected targets. This is similar to what you are basically saying. Yes? I'm good with this approach for AOO. Makes sense and it wouldn't even need prompts. You either set it for All Enemies and it works like Xcom Overwatch, or Selected Targets it works like Xcom Overwatch only for the targets you selected. Or you turn it off if you don't want it to trigger your reaction at all (like if you want to save reaction for a spell for your mage).
BUT... what about Absorb Elements. This exact same preset approach wouldn't work as well with this spell. Why? Because I may already be Fire Resistant (like if I'm a tiefling), so I may not want ALL elemental damage types to be triggered. If an enemy uses a fiery attack on me - let's say grease + fire arrow creates fiery explosion - my tiefling is already resistant. I wouldn't necessarily want to waste my spell slot on resisting fire damage.
Therefore, for Absorb Elements, the best preset solution would be something more like this: All Types/ Selected Damage Types/ Max Damage Potential > X (you set the value)/ Selected Damage Types AND Max Damage Potential > X (you set the value)/ Off. OR... at the very least, All Types/ Selected Damage Types/ Off.
In this way, with the fire resistant tiefling character, you could set your preset so that the reaction is only triggered on Acid, Cold, Lightning and Thunder (basically excluding fire). Or let's say you pick up a robe that protects you against cold. You could then set your preset to Acid, Lightning and Thunder (excluding fire and cold).
As for Max Damage Potential, the concept is that if I'm going to get hit by an ice attack that can do something like 12d10 damage, I might want to Absorb Elements, but if I'm hit by a 1d10 damage ice attack, I might not want to waste my spell slot. So, allow me to set a preset that says, "Ignore the baby Frostbite or Ray of Frost spells, but if I'm hit by Cone of Cold... YES PLEASE!!!"
And finally, what I'm saying is that for those people who want prompts all the time, allow the system to do that. Besides the presets mentioned above, add an option to turn on prompts. So, if they want to be prompted all the time, they can set all their presets to All Enemies and Prompt On. In that way, they can have combat interrupted every time any enemy triggers any of their potential reactions. But for those who want to rely on the presets alone, turn Prompt Off but set your presets to whatever you want. OR, for people like me, let me turn on something like "Selected Damage Types" AND "Prompt On" so that only if a selected damage type hits my mage will a prompt appear to ask me if I want to cast Absorb Elements. So, in my fire resistant tiefling example, if my tiefling was hit by a fire attack, no prompt. If hit by an ice attack, prompt appears and asks me if I want to use it.