Who or what do you anticipate will take over the role currently occupied by the United STates, @LennStar ?
The US role during the Trump years shows that they can stop doing everything, and things continue. There will always be someone ready to take influence.
I’m not disputing that. HOwever, that doesn’t answer the question.
It does, if you accept the answer that nobody HAS to. What obligatory thing is the US doing that must be done?
To be clear, it's not obligatory that the US (or any other single nation) does certain things, but rather that for the last several decades it simply HAS been the US.
Also, I'm not a proponent that the US *should* have this role... particularly not now, and not given the US's stances on international policy as of late.
A few things specifically (far from a comprehensive list):
World Reserve Currency. There's a few threads on this here and bogelheads, but business (particularly commodities) are done overwhelmingly on US dollars... and it's unclear how that could work without the dollar. The fact that the US is the only country which can create monetary policy on the sole reserve currency is not lost on teh Fed.
Internaional commerce, both shipping and air-traffic. Something like 80% of shipping traffic in internatinoal waters is overseen by the US. No other country (currently) has a navy which can ensure safe passage of ships across the globe. Likewise, almost all trans-oceanic flights (both commericial and cargo) are overseen by the FAA. Air Traffic Control, keeping track of flights, insuring international regulations has all been under the US. And we've mucked it up, too (see: Boeing). But which country(ies) take over this responsibility - and how - isn't clear.
Then there are all these international organizations. While I strongly disagree with our president on the manner of funding or the role of other countries, it's a simple fact that - as the worlds largest economy* - we supply a lion's share of the funding for NATO, the UN, (until very recently) WHO, IMF, etc. Partly as a result we arrogantly assume we get the most influence (and often we have). Again, I'm not arguing that the US should have these outsized roles, but should this wane, what happens to those organizations and their budgets? Will they shrink dramatically, or is there another country or group willing to replace the US?
bottom line: Every time the subject of the US no longer being an/the international leader there's a lack of agreement on how a world functions if/when the US no longer serves as teh reeserve currency, the overseer (often a bad thing) or international commerce, or the biggest pillar (again, often not in a good way) of so many internatinoal organizations.
Additional thought: Ever since WWII there's always been another country or bloc which were going to topple the US's international standing. Early on after teh wars many assumed Great Brittan would re-claim it's global role. Many then feared /thought the soviet union. Japan was almost certainly to overtake the US (according to most financial articles of the time) until their "lost decade" (which wasn't really lost). For the last 20 years it's been China... except now they might be more shaky than we realized. During the boom times the E.U. was also mentioned, up until the crises of the last decade, and future-thinkers have often India might get there.