I think cap rates are a decent proxy for that kind of thing, and they're not too bad recently in commercial office space, as far as I can tell. It seems that office buildings are selling at a discount to what they were a few years ago. But I suspect that many buildings have not yet changed hands, and thus have debt based on their old values.

I wonder how many underwater office buildings there are in the US right now, and who owns their commercial paper?

Edit: looks like about half of all office mortgages are underwater. Welp, there it is.

But the mortgage is fixed. Bringing people back in won't change the contract.

The value of the building is as a multiplier of the net operating income, that's what a cap rate is. If people come back in and companies expand and renew their leases, that increases the value of the building and reduces the capital needed to recapitalize the mortgage.

And, bringing people back in, especially in the context of e.g. Amazon with thousand of workers, also has lots of knock on effects on other businesses, and there are plenty of copycats who follow the big seven tech companies.

What is the motivation for the companies? They're not saving any money moving workers back in. They can just let the lease run out and not renew.