Even if unlikely not impossible. So we will be dependent on a few persons who have the code. Also this doesn't save all comments and discussions.
Why give away the sovereignty we have?
'Cause Phabricator is dying, see https://wildfiregames.com/forum/topic/42181-phabricator-is-no-longer-being-maintained-upstream/
Phab will keep running as long as we keep it running on our servers. Also I saw there were efforts to fork it.
Also sovereignty is essentially kept per the first point.
The first point doesn't keep our sovereignty at all.