Aside from the fact that IA64 wasn't a good match for IA32, it also wasn't that great an architecture on its own. It was very much designed to be implemented like the P4, with deep pipelines and a lot of control logic that figures out what your code is doing and does things in parallel and speculatively in complicated ways, with the goal of making use of lots of functional units when your programs are single-threaded. But then the x86 went the opposite way, towards more processors (and more cores, and even pretending you have more cores than you actually do). If your code runs best on a P4 than any other x86, it would be even better in IA64, but nobody's code is like that.
There's nothing terrible about designing an architecture that's just different from the other architecture the chip supports, so long as the new architecture isn't nuts. If arm64 is a different architecture from a company that knows a lot about a certain sweet spot from their 32-bit architecture, that's a whole lot better than IA64 being a different architecture from a company that'd been driven temporarily insane by their 32-bit architecture.