Wouldn’t you just take the depreciation now and write it off on your taxes if you own the building?

These people don't "own" the buildings though. Most of them have mortgages and if there's not enough, or no, tenants then they can't make their mortgage payments, they go into default, and then the Bank is left holding a property that's worth a lot less than what it was in 2019. This wouldn't be a problem if it was a few buildings in a few cities but when it's tens of thousands of properties in every city in the United States...well...we could watch a replay of the 2008 Crisis and it could be even worse.

If you are the lessor, why wouldn’t you keep pushing WFH so you can get a lower rate if you decide to renew the lease at expiration?

A lot of lessors are also owners or they have investment in CRE. Meta for instance may not own one building it's in but it could easily own another one in that city and they DEFINITELY own large expensive buildings in other cities! They have a strong financial interest in not allowing a CRE price collapse, it'd cost them way more money than they'd ever save negotiating down a lease rate on a rental property.