Well, it strikes me that those roles are different than the Chair in ways that are relevant to term length. None of those positions are elected, nor do they have a constituency to which they are meant to be responsive. In fact, we don’t really want them to have a constituency; they are purposefully insulated from political pressures in order to more effectively carry out their duties (e.g., neutrally interpreting the law rather than responding to the demands of popular politics in the case of the High Court). Hence, indefinite appointments without any need to rely on the political process makes sense for those roles. In contrast, the Chair is elected (even if by resolution) and does have a constituency to which they must be responsive, namely the Assembly. To be sure, the role of Chair may not be as overtly “political” as that of Prime Minister or Delegate (though it’s not clear that it couldn’t be), but a similar principle holds. Where an official is elected by and accountable to a constituency, it makes sense to require that official to re-establish their popular mandate from time to time. That their constituents could theoretically throw them out of office whenever they wanted doesn’t seem to have the same effect as the mandatory accountability mechanism of re-election. After all, the citizens could throw out the Prime Minister whenever they wanted in a vote of no confidence, but that isn’t really a substitute for regular PM elections.
All that said, this isn’t a hill I would die on. And you may be right that it’s not likely to matter much, since the Assembly will probably put a term in any resolution they pass adopting a Chair. But just wanted to explain my views.