The issue is that there’s nothing that requires prosecution, just allows it.

This is the doubled edged nature of prosecutorial discretion.

Well, in the idea of the system there is: The system is built around the assumption that Congress would impeach a president who fails to to the right™ thing (or fails to make his administration do ...)

However once the legislative branch surrenders oversight over executive there isn't much left keeping the system in balance. Even if judicial branch would call a measure unconstitutional, who'd execute that ruling?

The system is built around the assumption that a notable part of the system wants to keep it alive.

It's just as useful and effective as the international law and order that was setup after WW2.

So nada.

As long as you build an order around independence of countries and diplomacy (instead of, say, force) any organisation will only be as useful as countries are willing to follow and any structure can only be as good as the ones in charge are willing to go.

In consequences there are many flaws and a lot is stuck in post WW2 thinking, but I doubt there is a realistic chance of anything overall better.

The current U.S. administration tries to reshape things by disruption, we will see how this goes, but I doubt this will earn trust and buy-in from others. Thus not lead to a stable and "better" system. (While better, of course, is not globally objective, which again is key to the problem)

Would you say that was always the case, or just a more recent development?

Our entire civilization has always rested on a tacit "it's nice to have civilization, so we play by the rules" by everybody involved. Voluntary restraint is what keeps us from being animals, not nature or laws.

As Hobbes wrote so eloquently, we keep that compact because the alternative is "continual fear, and danger of violent death; and the life of man, solitary, poor, nasty, brutish, and short"

We're currently exploring how many of those rules are really necessary, and, as a society, have decided to mostly shrug off that exploration.

That is the part that's changed. A willingness to ignore the rules by some, and a collective shrug by most.

I think it's partly the way the US state is set up. When the president picks the Supreme Court judges and they have lifetime appointments you don't truly have separation of powers. Then the whole thing is meaningless. When you have "liberal" and "conservative" courts based on the make up of the judges you need to start again.