Why would they do that, when what they did with ARMv7/ARMv8 was perfect: treat them as separate ISAs, and allow for cores that either run both or run ARMv8 only. Why would they need to define a "transition period", rather than letting the customers decide what's best for them and their market?Maybe, maybe not.
For one thing we have no idea what the transition plan for v9 is.
They COULD have a temporary period during which devices can decode both v8 (hopefully 64-bit only!) and v9 instructions. How feasible this is depends on how ambitious they are in redesigning v9.
Since there's essentially zero commercial (closed source) ARMv8 binaries, an ARM server vendor could quite reasonably go ARMv9 only from day one. Why should they pay the price of a transition period? Why should Android OEMs that serve a market that goes as low as sub $50 phones be forced into a transition before it makes sense for them?
I'm still skeptical there IS an ARMv9, since AFAIK the only evidence was a job posting mentioning that. That might be something they are targeting for 2025, and we'll see several more ARMv8 iterations.