I think that it could be pretty reasonable to have a set period every few years, either 5 or 10, where we officially evaluate ministerial roles. Basically a set time for a cabinet reshuffle, for mandates to potentially change, and for official retirement due to length of service. At the very least, I think we should establish that people will be expected to step aside for a term every once in a while. Term limits don't really make great sense in the context of potentially immortal leadership, because what if you're at war and actually want your best general back, but he's already lead the fight for a while?
Despite this, I think there is a fair point to try and establish ideas like taking a "retirement" every few years, and what that actually means in practice. Do top civil servants get any form of old-age pay? Maybe establish that after something like 25/40/50 years (some year number like that, maybe a siding scale?) you get retirement pay for 25 years, in which period you're expected not to serve in government or be at the top of industries. Maybe the longer you serve, the longer you're expected not to serve with a potential for emergency override?
I think there is a fair point to trying to establish norms like this. Additionally, I think establishing that we actually may take actions proposed by people questioning us is important so the institution feels like it actually provides value.