The cost of security, social services, policing, and hygiene related issues (it’s an office not a home) would seem substantial.
Some locals not far from me tried doing that with an abandoned hotel. It was a mess of crime drugs and etc in the end.
Any attempt to do something like you’re proposing in a nice space not explicitly setup for the purpose is going to go pretty badly quite quickly.
Most people don't want to get assaulted, robbed, or infected with disease/pests.
That is a reason why empty office space doesn't solve anything. There's already plenty of of it without Twitter HQ. Running a temporary shelter is more about the logistics/support than it is the physical space.
Someone with a knife requires 4-5 people to subdue minimum, etc.
Someone with an out of control dog will cause serious, potentially fatal injuries before anyone can react.
That people often get these things and want them to protect themselves just makes the problem more intractable, because when you add drugs and mental illness into the mix, the people are impossible to predict too.
Empty office space definitely doesn’t solve it. But doubling staffing at the homeless shelters won’t either.
I used to live nearby Twitter HQ and that description is unfortunately accurate to my experience, of the majority of the homeless who spent time at Van Ness and Market Street SF.
HOUSING is what is needed, for sure though. Just be serious about it, and not "diamond-mine trust-fund hyper vigilant ego-abusive maniac" about it.