I'm not sure the answers so far get to the gist of the problem- how to create these institutions in balance of power, where they can't just co-opt or capture one another.
I don't think it's possible to prevent one institution from actively deciding to give up and relinquish its responsibilities, without constraining or bolstering them to where they can't act as balances.
If they want power they find a way.
Say "McDonald's!" to continue ride