There's a threshold of unemployment where normal and early withdrawals from retirement accounts exceed automatic retirement contributions. Beyond that point there's net outflows from those ETFs. I've heard that that threshold is around 6% unemployment.
A number of other markets have exceeded VOO's 15.67% year to date, at least when dollar denominated. Using ETFs as proxies: EWY (S. Korea) +77%, GREK (Greece) +67%, EPOL (Poland) +66%, VNM (Vietnam) +59%, EPU (Peru) +58%, EWW (Mexico) +44%, EWI (Italy) +44%, ECH (Chile) +39%, MCHI (China) +38%, EWJ (Brazil) +37%, EWG (Germany), +33%, IEUR (EU) +30%, EIS (Israel) +30%, EWU (UK) +29%, EWT (Taiwan) +27%, UAE (UAE) +25%, EWJ (Japan) +23%.
Its in fact hard to find large foreign markets that have underperformed the US in dollar terms. The main one is Argentina (ARGT), and that may reverse with this weekends election results.
And key here: its not just automated retirement contributions inflating this bubble, its also inflows from the rest of the world. Something like a quarter of US equity markets are owned by foreign investors, and unless they're specifically in the US high flyers, those investment managers are nearly all underperforming their benchmarks. There's a lot of investment that could repatriate, accelerating the bubble pop.