I don't think Amazon is making a good decision here, but what makes you think the existing workers who are happy to work from the office would not be willing to

I don't think Amazon is making a good decision here, but what makes you think the existing workers who are happy to work from the office would not be willing to work from home during a crisis if asked? A global crisis necessitating a transition to working from home should not immediately result in a company like Amazon needing to substantially hire more staff, in fact, that is probably a big part of what got them into this situation.