OK. I think what we're saying here is that Reactions COULD be done as more of a Scripted/Preset Reactions. However, if you really want Reactions to be done so that players have full control over them, you need popups to occur when a reaction is possibly triggered. Otherwise, with any kind of Scripted/Preset Reactions there are absolutely going to be times when your characters react in ways you don't want them to.
Even now, with AOO being a Preset Reaction, there are times my characters use an AOO when I'd rather they didn't. Baby goblin comes running past Lae'zel with 2 HP left, and Lae'zel uses her Reaction to hack him to pieces. But, a moment later, the ogre I'd rather have her AOO on takes off running right up to my wizard and smashes him in the head. With the popups, I'd be able to reject baby goblin running past. Let him get to Gale. Who cares? He might do like 5 damage. Then I'd still have my Reaction to hit the ogre with 2d6+4 (and being the Blooded Greataxe it does more to wounded enemies) and possibly take down the already wounded ogre who has 15 HP (especially since Lae'zel has, at this point, Great Weapon Master). And, let's face it, if the AI determined that Lae'zel would get an AOO on the ogre, it might not try to run past her to get to Gale. Thus, by implementing the popup windows, it gives us, the player, more control and lets us make the call even on something as basic as AOO.
I'm just saying, they COULD do it with the whole Preset Reactions thing, but it won't be as good. There are going to absolutely be plenty of times when players get frustrated because their Reactions were wasted on stupid stuff. Yes. It'll slow down combat a smidge and make it flow less smoothly, but it will definitely give players more control.