The hardware upgrade comes with the purchase of FSD. They can't sell you something that requires further purchases just to function without disclosing that fact.
Yes in a way they can. Right now what you get with FSD is what you already had with EAP. The only new feature they promised is one day you get stop sign / traffic light recognition.
When more features get added, there is nothing stopping them from re-working the categories and adding something above FSD - call it Super FSD. They may even say you get the software free for SFSD, but you need to pay for 'HW4' or whatever they are up by then to use it. They never promised you that buy buying FSD you will get every new thing they come out with and hardware that can run it.
Anyhow if it is not in writing I would not assume that is what will happen. For the features they already have promised for FSD HW2.5 will work as they are the current EAP features.