DalvikVM is a 32 bit virtual computer (machine), while the VM can be ported on any platform, the virtualized machine always have the same 32 bit specs regarding is opcode and register, limiting apps on a virtualized 32 bit platform only.
To be clear, I agree DalvikVM and virtualized Apps should have some benefit from all the changes in the ARMv8 platform like any other apps. But Dalvik developers won't be able use AArch64 features explicitly.
I'm still not convinced that reworking DalvikVM for 64-bits is a monumental task. Perhaps even, the benefit might only be minimal...
(But I'll be the first to admit I don't know a lot about Dalvik :-) )