Agreed, auto success on a skill check nerfs challenges.
If the DC is so high that the PC doesn't succeed with a 20, it seems too random to give it to them.
Then again, it depends on the situation: a nat 20 trying to convince the penny pinching tavern owner to give you a discount seems like fun even if the DC should be infinite; but when dealing with something story related, I'd stick a little closer to the rules.