Mohamed El-Erian sees a potentially difficult path ahead for US markets and the economy. The top economist and former co-CIO at PIMCO said there are challenges facing American exceptionalism — the idea that US markets and the economy will continuously outperform the rest of the world in returns and economic growth. That's due to a vulnerability that's been lurking in markets in recent years, which spawned during the era of ultra-low interest rates and abundant liquidity, El-Erian told Business