>Large tenants leave all the time, it is a cost of business: you factor that into the lease terms.
Big tenants leaving is something that takes a lot of effort to manage. If a tenant is potentially giving back 100k sqft at the end of the lease a landlord is going to be getting in front of that years in advance by determining renewal probability, engaging with brokers to find tenants that might be in the market, and possibly actively marketing the space even while the tenant is still occupying the space. Even with that lead time, that space still might be vacant for 6 months to a year, and that's in decent market. This is all because larger tenants don't move at the same pace medium or smaller tenants do.
As far as being factored into the leases, the way WeWork structures their leases is such that the break even point for a the landlord is farther into the future that most of their typical leases. This is mostly due to the significant build out work that WeWork does their best to get the landlords to subsidize, and the amount of free rent they ask for on the front end. Therefore, if WeWork goes dark before year 5 of their lease, then the landlord has likely lost money on that lease.
When it comes to the landlord taking over the short term rentals, some probably will, some maybe won't. Who knows if the tenants will be interested in that or whether the landlords will make enough money for it to be worth their while, but these kinds of short term rentals are not the kind of business landlords want to be in.
For WeWork to leave a lot of space behind that means they are taking a lot of space elsewhere (or going out of business) Of course they can leave one space behind no problem, but that happens all the time. (though less valuable places may run into problems it was the same problem they had before). Large tenants leave all the time, it is a cost of business: you factor that into the lease terms.
If WeWork goes out of buisness that changes things, but those renting from WeWork need to do something, some will talk to the building owner about getting a lease on their current space, so it won't be as bad as you state (it won't be good either). This same can happen if WeWork decides to move: those who are using WeWork space may decide that the location is important and see about remaining in "their" space.
It will be interesting to see what happens.