Think of it from their point of view though. They want to try and do something. For me to just flat out tell them “no, there’s no possible way” is discouraging and robs them of autonomy. Obviously for crazy extreme circumstances I won’t let them, like “let me convince the king to abdicate to me!” type things. But if I think the DC should be 25 or something I’m not gonna bother wasting my time calculating what the theoretical maximum could be for the roll because I genuinely cannot know. The player can always do things I don’t expect or use other players’ things to help. For reasonable but implausible things I’ll allow rolls even if a nat 20 wouldn’t work because I’m not calculating what a nat 20 could theoretically be.
Plus, I often give people little flavor benefits for nat 20s even if they don’t have mechanical success.
Think of it from their point of view though. They want to try and do something. For me to just flat out tell them “no, there’s no possible way” is discouraging and robs them of autonomy. Obviously for crazy extreme circumstances I won’t let them, like “let me convince the king to abdicate to me!” type things. But if I think the DC should be 25 or something I’m not gonna bother wasting my time calculating what the theoretical maximum could be for the roll because I genuinely cannot know. The player can always do things I don’t expect or use other players’ things to help. For reasonable but implausible things I’ll allow rolls even if a nat 20 wouldn’t work because I’m not calculating what a nat 20 could theoretically be.
Plus, I often give people little flavor benefits for nat 20s even if they don’t have mechanical success.