The tricky part here is that "efficiency" is not a single dimension! Transformers are much more "efficient" in one sense, in that they appear to be able to absorb much more data before they saturate; they're in general less computationally efficient in that you can't exploit symmetries as hard, for example, at implementation time.
Let's talk about that in terms of a concrete example: the big inductive bias of CNNs for vision problems is that CNNs essentially presuppose that the model should be translation-invariant. This works great — speeds up training and makes it more stable – until it doesn't and that inductive bias starts limiting your performance, which is in the large-data limit.
Fully-connected NNs are more general than transformers, but they have _so many_ degrees of freedom that the numerical optimization problem is impractical. If someone figures out how to stabilize that training and make these implementable on current or future hardware, you're absolutely right that you'll see people use them. I don't think transformers are magic; you're entirely correct in saying that they're the current knee on the implementability/trainability curve, and that can easily shift given different unit economics.
I think one of the fundamental disconnects here is that people who come at AI from the perspective of logic down think of things very differently to people like me who come at it from thermodynamics _up_.
Modern machine learning is just "applications of maximum entropy", and to someone with a thermodynamics background, that's intuitively obvious (not necessarily correct! just obvious) –in a meaningful sense the _universe_ is a process of gradient descent, so "of course" the answer for some local domain models is maximum-entropy too. In that world view, the higher-order structure is _entirely emergent_. I'm, by training, a crystallographer, so the idea that you can get highly regular structure emerging from merciless application of a single principle is just baked into my worldview very deeply.
Someone who comes at things from the perspective of mathematical logic is going to find that worldview very weird, I suspect.
