I think there's also an element of moving goalposts. Maybe we understand certain structures of a fully connected nn, but then we need to understand what's happening within a transformer.

They're going to continue to get more complex, and so we will always have more to understand.

I think what you're hoping for is some theory that once discovered will apply to all future NN architectures (and very likely help us find the "best" ones). Do you think that exists?