I can totally understand the prompt only argument. It IS a turn-based game. By having prompts, you actually get to DO something during enemy turns. The preset auto reactions system means you just watch.
Part of the point of the reaction system to begin with IS that you get to do something during enemy turns. You aren't just helpless as you watch them.
I have done several Solasta battles again since discussing this - characters at level 10. It's really not annoying or bad. The number of prompts is usually quite minimal when based on reactions only.
So, even if they did no presets, I'd be happy with the Solasta reactions approach. It really isn't any different from a true Tabletop experience where the DM says, "Hey John. Goblin 4 is casting Fireball. John! Stop watching TV for a sec. Goblin 4 is casting Fireball. Wanna Counterspell?"