If global equity funds can be relied on to diversify with well performing companies no matter where they are located then it doesn't seem like bad advice to invest in such a fund no matter how the US economy performs. Especially for those who aren't interested in researching deeper into investing.
Returning to the title of the thread, I think there won't be a sudden collapse that will change a generation but a thousand little cuts over the coming decades to things we've come to take for granted in our standard of living since maybe the late 90's. As we've already seen happening for some time things that didn't use to cost anything will have a price, things that were affordable and commonly enjoyed will become more and more co-modified that they will all become luxuries reserved for special occasions.
My concern is things like demographic changes and cost of living challenges in to the future will put brakes on stock market returns. The last 10 - 15 years will be viewed as a golden age for global wealth creation, pension fund growth, generous government hand outs that never used to exist. And this era will be seen as wasteful because it could never last, that it became more and more irrational exuberance built on a house of cards.