Two things:

--Technically there is no such thing as 'borrowing' stock. That's just a colloquialism employed. The contract used, when shorting a stock, is for purchasing X shares with an agreement to resell back to the original owner.

--It's not entirely clear that the market is wildly overvalued. The economic slowdown has, first, been the result of a complete externality: there is no real problems with resources, inventory, supply, cash, market failures, or other such intrinsic mechanism. By way of contrast, consider the 2008 debacle where a combination of broken lending, obscure packaging of securities, and insanely over-leveraged financial institutions combined in a shit-storm of fail, to contribute to a near-meltdown of the whole shebang. Secondly, the economic slowdown has been deliberate, transparent and even occasionally well-managed (particularly on the state level). Third, while slow at times, the commitment to stimulus and relief from the Federal government has prevented panic. And, fourth, because of the preceding three points, a quick and robust rebound-n-recovery is widely expected once the pandemic is tamed.