The only thing that can stop compatibility is the lack of 5Ghz, and I haven't seen anything yet that says O4 can't operate only on 2.4GHz. The constraint might limit some O4 performance, but it's not a outright showstopper.
Also, there isn't any reason at all the
A3 can't be updated in the future to support O3(+) to provide compatibility with the RC Pro.
Both the RC Pro and the
A3 are implemented with
Software Defined Radios (SDR). There is very little from a hardware standpoint that is specific to the transmission protocol... It's implemented in software. This is the reason the various RC's can, and have been updated through O2, O3, O3+.
Whether or not an SDR-based RC gets updated is a marketing, product roadmap, and ROI decision. It has nothing, usually, to do with NEW TECHNOLOGY, which isn't magic.
It's simply engineering.
It's my
opinion that, given how DJI has positioned the RC Pro in their product ecosystem they will make the investment to support the
A3. It may take adding 2.4GHz-only operation to O4 (which I'll bet is already in the spec), with an update to both the RC Pro and the
A3, but I'm rather strong on the idea it will.
It likely will require patience, however.