Microsoft is a lot more aggressive with EoLing it's Windows versions now exactly because XP lived so long. It was an absolute pain for them to maintain and support that for so long and they've made very sure they don't repeat that experience.
Yup I've got a box in my mum's house that all my off site backups go to and it's a damn site cheaper just to give her some money for the electricity cost of it each month than pay for any cloud service.
This doesn't remove security and compliance requirements for the business though. For our Linux endpoints we still deploy an AV on them and limit the user's ability to add exclusions.
If those are important to you yes but the old desktop route generally means you'll get access to more powerful chips plus the ability to add more storage and memory.
At a functional level I can't see any reason why it wouldn't work. Though my preffered method for that has always been old desktops over phones as it gives a lot more flexibility overall.
Generally speaking I find USB hubs with ports on them work pretty well with android as they're just relying on the Linux kernel drivers for the USB support.