Ooooh, I’d like that! Well, there’s 3 parts to the (random user input / scripted game output) conundrum:

1. I think it is fair that if you ask, ‘Why didn’t you say something?’ the NPC might either respond as if it is being accused of sabotage, answer the damn question, lie, or prefer not to talk about it (it’s personal).
2. I’d keep a short list of standard options – probably in a collapsed scroller kinda thing so you could either verbally say or type whatever you want, OR you could click an arrow to pick from a list. That way lazy or stuck players wou;dn’t have to think of all the options, and players interested in roleplaying could do as they please.
3. I’m OK with, “I’m not going to respond to that”. I’d hope each character had several variations of that, but I think it is legitimate for NPCs to dislike being pestered. Shopkeepers might have replies like, “Are you gonna buy something or are you just here to bend my ear?” or “I don’t see how that relates to my inventory.” Random townies might reply, “Do I even know you?” or “Would you PLEASE stop bothering me.” or “You’re harshing my mellow, man. Shhhh… Just chill.”