Why does macports require a migration across major version bumps?

[db]

On 30 Aug 2017, at 20:29, Clemens Lang <cal at macports.org> wrote:
> Use 'sudo port -f selfupdate' instead, which should also recompile your
> version of MacPorts.

> In theory, reinstalling MacPorts and using 'sudo port -t upgrade
> outdated' after an OS update should thus have the same effect as the
> migration instructions.

Running these two commands would then leave you with a working system, except for those arch-dependent ports?