VDI and virtual environments have been around for a while, many orgs had non-persistent desktops or multi-user environments for decades.. users logged off or were forces to log off at night... how did they save their info each night???
How do you manage a persistent desktop environment that never goes down??? when will the user log off if you told them they can leave it on forever? I see VDI/DaaS as a "Desktop as a service" and that means it is there when you need it but at the end of a time (can be one hour, a day, a few days) you return it, "I" destroy it, rebuild it, polish it, update it, make it run perfectly, and you can use it again.. as a service.
persistent is like the FAT clients in the 80/90s.. not very flexible and too much local data..