PIMCO’s Mohamed El-Erian says global central bank policies have inflated asset values beyond what fundamentals merit, and says that won’t last forever.

“That central banks are essentially ‘all in’ is, in the short term, good news for all types of markets,” El-Erian writes in a piece for CNBC.com. “Yet … investors should not get too carried away. There is a limit to how far and how long prices can deviate from fundamentals. This is particularly the case when central banks, acting without the support of other government entities, do not have sufficiently-refined tools to secure good and sustainable economic outcomes.”

El-Erian says that “investors’ romance with the ‘central bank put’ should not be unconditional or everlasting. Moreover, it needs to be accompanied by significant portfolio differentiation, responsive management of overall risk exposures, and positioning that also reflects more durable global themes.”

In the end, El-Erian says, either fundamentals will improve, or asset prices will decline. “Which outcome we eventually see depends in large part on whether other government entities finally step up to their policy responsibilities,” he says.