I'll tell you when they come begging to China.
Begging for what, exactly, and when? Anyone can predict events after we're dead.
The US is unlikely to simply collapse. Over the next century it may suffer a relative decline. But it has huge advantages to keep itself afloat. Much like people have been declaring China is going to collapse, the same is done with the US every so often. For the forseeable future it's wishful thinking. Moreover if the US did go down suddenly then it would drag most of the global economy with it, which would also be really bad for China.
China, South Korea and Japan could at some point enter into a trade compact, or China might be able to join the CPATPP. But it would require significant changes in how China wanted to conduct business and resolve trade disputes. Depending on how close some sort of impartial third-party arbitration would be required, which currently Beijing would not allow as a point of principle.
An EU-style political organisation would also be impossible because currently seats in the parliament and most votes in the council are related to population size. They'd have to be allocated based on the number of member states (e.g. 1/3 for each country if China-ROK-Japan), and I'm sure China wouldn't want a situation where it could be outvoted, which might require vetoes as ASEAN has.
Countries that can't live by their own means in order to maintain a modern lifestyle need to export their goods.
That would include China, which has a huge trade export surplus. What's your point, exactly? Is China going to impose trade embargoes against countries that don't sign close trade pacts? A lot of countries trade happily with China without detailed FTAs.
