ARM launched aarch64 only designs back in 2019, it is already happed, not "going to".
You need to read more carefully. It is totally correct that they do have the Cortex-A65 core, which implements only the AArch64 execution state of the ARMv8 architecture. Nothing inherently wrong with this - in particular since Cortex-A65 is targeting embedded systems. They also have the Cortex-A32, which implements only the AArch32 execution state of the ARMv8 architecture. You could conclude from this, that also AArch64 is optional, just because such a core exists.
The question was however, if the implementation of AArch32 is optional in the ARMv8 spec. So an implementation compliant with the ARMv8 spec cannot drop AArch32 (and neither can it drop AArch64) - as both are not declared optional - at least to my knowledge. This holds in particular, since the ARMv8-A reference manual explicitly says that optional features are explicitly declared optional in the document. (i already did cite this part of the spec earlier).
The 8000+ page architecture reference manual has over 30 references to "AArch64 Only" in the first 100 pages alone.
Thats a good one. The term "AArch64 only" are used in the spec, whenever there is a feature, that is only available in AArch64 execution state. Surely there are some features only available in AArch64 state and others are only available in AArch32 execution state.