What does a PC's ability modifier have to do with whether or not the DM grants auto-success in a situation?
I can only speak for myself here.
But p 58 of the Basic PDF says that "The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure. When the outcome is uncertain, the dice determine the results."
So here's an example, beginning with some autobiography, to show what I have in mind.
On a particular walk that I do often with my partner, there is a place where I do some jumping - a standing jump from the ground up to a higher surface - just to try and keep that part of my leg strength up. I have to warm up a bit to do the jump - normally it is the third or fourth try where I can land on the high surface rather than just land back down on the lower ground.
If I were a competitive sprinter or jumper or volleyballer or similar, then the jump would be utterly trivial.
In 5e D&D, the difference between my ability to jump and the ability of a competitive athlete for whom the jump is trivial is expressed in differences of STR score and differences of Athletics proficiency/expertise.
Thus, to me at least, it stands to reason that a decision about what can be automatically achieved might depend on the PC's ability modifier.
Mutatis mutandis, the same would apply to attempting to find or recover water in the desert, or attempting to sell someone the Brooklyn Bridge, or any of the other various actions that a player might declare for their PC.