Even when a single server can deal with the load of an entire country, having only a single server each for IPv6 and IPv4 potentially also raises a question of resilience through diversity: Even when a client were configured with multiple numbered subdomains, and able to use both IPv6 and IPv4, it would still end up with a single actual server only, behind two IP addresses.
I have a case where I have the only server in a zone, at least during parts of the day, for one IP family. I’ve been considering removing my server altogether for that IP family to allow clients to access a more diverse set of servers for that family, namely the enclosing continent zone (when there is no server in the country zone), rather than just getting my server, plus sometimes one, two, or so other servers.
But that zone still has slightly more than a handful of servers of the other IP family to provide some diversity when both families are used. And when I remove my server, then clients would probably not see much of a change because the other servers of the same IP family phasing into and out the zone will often also be alone when they are in, so I think it might be slightly better for diversity for me to leave my server in place.
I would be curious at to whether leaving a server in a continent zone, and the global zone, but removing it from the country zone, would be possible. That might be an option here, and might also be helpful for various other underserved zones, or to help servers overwhelmed in their zones - if the respective operator so chooses.