[asavah]

the ARMv8 performance benefit is not significant (from past testing, 10-15% or less compared with ARMv7),  

significant is such a subjective term. I'd call a 10-15% gain from software with no change in hardware very significant.
People pay hundreds on a new phone, or thousands on a new computer for a 10-15% hardware CPU gain. To get that from nothing more than software on existing hardware, is truly significant.  

 All the features of rpi userland can't be built for 64bit atm (the most important being *GL and OMX/mmal), because vc4 is actually 32bit and making userland fully compatible and working with aarch64 would require insane amount of time and tons of elaborate hacks,
and AFAIK this is not a priority for RPF.
10-15% gain of what?
ATM Kodi-rpi won't work (or even build) on a pure aarch64 OS, so there is nothing to be gained.
There may be a generic approach by using mesa vc4 GL drivers and Xorg, but currently in such configuration there would be no hw video decoding, which again makes it almost pointless.