You're assuming that "different blockchains" is a viable concept that's sustainable in the long term.
I know many people are wishing very hard for this to be the case, but economics doesn't care about how hard people wish...
The point that AT is about is that it is *blockchain* agnostic (i.e. it doesn't care which blockchain you use).
If we end up with *only one* then AT *still works* so I don't quite get your point (*economics* is not going to *get rid of AT* for some unspecified reason when I've just pointed out that multiple blockchains are not *essential* for AT).