dav1d is still in the "some very-important software" group; the vast vast majority of software doesn't and will not write everything in assembly. If you think RISC-V is gonna in any way change that, you're.. just trivially simply plain wrong, 99.999% of people will not bother learning assembly regardless of how good of an idea you think that'd be. (never mind that even if more people learned assembly, 99.999999% of said assembly would be full of system-killing bugs)

RVA23 doesn't in any way ease porting, no clue what you're on about there; besides vectorizable code, where you necessarily do simply just need RVV or similar to get good performance, base RV64G does just cover everything needed for software to be able to run. All RVA23 does is just provide a baseline with extensions to be able to achieve good performance, and cheap instructions that software should've been already utilizing for decades but hasn't generally been able to due to legacy hardware not supporting them (importantly clz/ctz/cpop, but also to a smaller extent min/max, zicond, bit rotates).

Granted, RVA23 does have some more questionable (though still actually useful) inclusions like the cache block ones that force 64-byte cache lines to not be dysfunctional, but that also brings up the massive wart of mandatory 4K pages in base RISC-V, not even RVA23, that's explicitly chosen for ease-of-portability.

Maybe RV64 is the end-of-line for CPUs, but, for all we know, RV64 today might be what Intel 8086 was in 1978, and RV64 is to be extended and grown and eventually just replaced in the future.