Good question, my original intent with this rule was to make the amount of times the dungeon could be explored limited, because of the *dank* treasure, it could easily be exploited if it was infinitely explorable. Revisiting this rule, there are probably much better ways to implement it, and i think your suggestion of making it some sort of range is better, especially with the +1 modifier, because then it becomes a probability thing. It should actually be something like...
Roll a d20, on a 17+ the gate crumbles, also add +1 each time they revisit the dungeon.
This would make it so on first visit if they rolled a 17-20 it crumbles, and the next visit 16-20, then 15-20 etc etc. This increase the probability of crumbling each time in a way that actually makes sense.
This could also be flipped so that a LOW roll causes the gate to crumble, which is more inline with how lots of systems treat failure, but i feel like the math would be slightly more complicated, and i always try to make my games easy to grok.
I'll make sure to revise this when i update the pamphlet.