My point is it changes the dynamics of DM Fiat.
Before you could objectively decide the difficulty of an action and let the dice fall where they may. You only had to deny an attempt on the case of possibility
Or in other Terms,when a DM is potentially deciding on a check, they are thinking about 2 things: Possibility and Probability.
Possibility is the ability for the character to even succeed at the action.
Probability is how likely they can succeed at a possible action.
Usually you think about if the action is possible, usually by examining proficiency, equipment, or other parameters. Then you set a probability based on a difficulty you gauge if you deem the action possible.
Sometimes possibilty and probability are linked. An action's scucess might be only of pure difficulty or experience. In these times you can merge Possibility and Probability and set a DC. A DC over 20 is only possible for the talanted, skill, or enhanced as you cannot roll higher than a 20 without a modifier. A DC over 20 also had a low probability of success. PCs do to their equipment and magic can high themselves up their probability of success and turn the impossible to possible.
Automatic Success on a 20 wonks it up as it puts more work on the DM by enhancing the Fiat of when an actions possibility chances via a chance of probability.
It's more work for a DM in a game that already piles work on them.