They can't hold landlords hostage. The best they can do is to cancel the lease and convert WeWork into an Airbnb style platform that lets the landlord rent out their office space as a workspace. This shifts most of the risk to the landlords and WeWork can simply take a transaction fee as a middle man.
Landlords don't want to do that, or else they wouldn't have had the need to rent to WeWork in the first place. Landlords like the risk profile of having an entity in the middle. There are significant operational costs of doing this, and in some cases there may be legal or tax provisions that prevent a landlord from doing so.
