[ Just like you can’t climb a flat 20 meter wall without equipment, even if you have Athletics 4 and Strength TN 12.
I agree with you, saying there's no need to roll is always a possibility.
But then the question becomes when do you say it ?
If you consider the limit is 20 meter, then climbing 15 meter is possible, you roll, and it's easy for some characters. You just add 5 meters and it becomes impossible for the same character...
The system should be more granular, with probability of success decreasing step by step down to 0, not changing from "easy" to "impossible".
As hsi379 has demonstrated, the probability of success for characters with high skills is always high, if you allow to roll.
I think there could be a table stating malus dice or malus on TN according to the difficulty, like the Difficulty Class in D&D5, ranging from DC 5 Very Easy to DC 30 Nearly Impossible, by steps of 5... Even if DD5 is not my favorite, at least the players and the DM have clear and shared targets...
The issue is not really addressed in the Core Rules, unless I missed it.
And I don't see why a malus on TN is impossible, as it is clearly used in Combat : you raise the Strength TN with the Parry Rating of shields as it is harder to hit someone with one.
Then climbing a 20 meter wall is harder than 15 meters, you could raise the TN with a "Difficulty Rating"