I've been in situations like this. I'll give a simple rule set of some topic that someone hasn't quite managed to defragment in their brain (and thus know it as

I've been in situations like this. I'll give a simple rule set of some topic that someone hasn't quite managed to defragment in their brain (and thus know it as a bunch of rules and special-cases), and then they'll spend hours going "But what if <x>?". Eventually they believe me that my simplified model is equivalent to their overcomplicated internal model. The result is usually closer to "You're right; it is that simple." but that's fairly similar to "It's trivial." In short: Often people need to try a lot of "Yeah, but what if <x>?" before they satisfy themselves that something is in fact as simple as it appears.