Windows XP no longer has support, and thus no longer has proper safety, fixes, updates or anything that kept it viably safe to use. With an radio system, if it no longer has support, that means no more new parts, updates, "good tech support" , replacement radios etc... As there are so many of these systems out there, the pool of spare parts is going to go away very very fast. You should already notice that formats that are not fully supported anymore, are not on the market anymore radio wise. Show me a brand new in the box ASTRO SABRE, or other of the "Jedi" radios? Also remember, its not just the Smart Net, and Smart Zones going away, Harris is ending support for the entire EDACS Lineup as well. Now if someone wants to keep using the Smart Net/Zone systems, they have a vendor they can go to, EFJ/Kenwood, who has now got the rights to the technology. Though they will be pushing their own P25 line as hard as Motorola and Harris do theirs....
But back on my original point - Why does an agency/municipality/county push hard for a radio system to be built, then not move to even use it partially?