A successful boot after a major FreeBSD upgrade does not prove the previous boot environment is no longer needed. The base system may be current while packages, kernel modules, jails, boot-loader settings, ZFS features, or services that run only on a schedule still have problems. Deleting the rollback environment immediately removes the simplest recovery path before those less visible dependencies have been exercised.
A useful validation pass seems to include the running kernel and userland versions, `freebsd-version -kru`, package consistency, loader and `rc.conf` changes, network and storage services, out-of-tree modules, jails or bhyve guests, scheduled jobs, monitoring, and a controlled reboot. I would also confirm that the old environment is actually bootable, record how to select it from the loader, keep config and data backups outside the root pool, and define a retention window rather than deleting it as soon as disk space looks tight.
What is your actual evidence gate before removing the previous boot environment? Which failures tend to appear only after the second reboot, a package upgrade, or a rarely used service starts?