I agree for most expected scenarios: regular updates via sysupgrade, nothing goes wrong, stable sysupgrade process. In this case the initial recovery instance is never used or needed again.
But: I think sysupgrades between major versions are mostly done only between one major version to the successor major version: like 22.03 to 23.05. There will only be a small to zero number of users sysupgrading from let's say 22.03 to 28.xx.
If the recovery instance is needed some years later it should be safe to upgrade between each major version to the successor major version. But upgrades directly from a release several years ago jumping over intermediate versions is not tested and not guaranteed.
For now years old EOL sysupgrade images are still available on https://downloads.openwrt.org but this is not guaranteed in the future.
I will keep one sysupgrade image of each major version for my E8450 by myself. With this I am independent of the OpenWrt EOL sysupgrade image archive and the recovery instance can stay on 22.03.
A better solution would be if we could easily update the recovery instance once for each major version.
In practice for the expected lifetime of let's say five years no recovery instances updates should be needed. But when I see that some models like the TP-Link Archer C7 v2 are supported since 14.07 and have no projected device EOL I am not sure if it is the best decision to stay on 22.03 for the recovery instance forever.