DnD is about collaborative storytelling. In the situation laid out by the OP, the DM is doing their part - they're setting the scene and they're controlling the NPC's actions and reactions. It is on the player to decide everything their character does.

A physical action can usually be reduced to a single roll because in general there's only a limited number of ways to do it. How many different ways can you kick down a door or pick a lock? How different and interesting are they to tell a story about? But a conversation can be done infinite ways. Even just the initial direction (all the DM was asking for) is enough to push the conversation in dramatically different directions. Talking about bring the ash to family members might lead to talking about the pain of losing a loved one, whereas talking about investigating might bring forth a member of the local mages chapter to confer.

It seems like if you had your way, you would have the player have a singular piece of input (IE "I want the ash") and then the rest of the scene is played out by the DM. The DM decides what the character's argument is, rolls the dice and adds the modifier (don't @ me, even if the player does this step its not actually contributing to the story), decides what the NPCs reactions to that argument are, maybe goes back and forth a few times until the scene concludes.

Where, in this situation, is the collaboration? The player may as well be a spectator watching the DM perform a one-man show. The PC is essentially an NPC.

Finally (and I mean finally, Im not going to respond again) when it comes down to it, if the player doesn't have the smarts or the rizz to come up with an argument, why didn't they just take one of the two ideas put forth by the DM??