By that theory then, shouldn't all ILEs be 538? My Ego is
and I'm a 7, not a 5. And then you might as well say all SEEs have 8 as a core type, and all LIEs 3, which is obviously not the case.
Enneagram is mean to predict motivation/reaction in a stressed environment. It's more helpful on a personal level so that you can figure out what you need to work on personally. It doesn't do the best job at predicting behavior. It isn't meant to help with relationships. It's very much a personal development tool. I think that's why it seems so "simplistic". People look at it wanting to know every facet of themselves and their relationships with others, and that's just not how it's meant to be used. People of the same type can present very similar and very different behaviors, so people look at them and think, "Wow, this obviously isn't accurate if I'm a 7 AND this guy here is a 7. We're so different!" and then feel the need to over-complicate things with subwings and tritypes and etc.
I also like to think tritypes were made more popular by the enneagram institute so they could make people pay to go to their workshops, but that's just me being cynical.