Cell phones won't replace radios anytime soon, if ever.
Radios are quicker and easier to use, as well as have the benefit of broadcasting transmissions to multiple receivers at the same time. That's crucial in public safety when you have more than one unit that needs to respond, or if only one unit is responding, everyone else is aware of that unit's assignment should they need backup.
A good radio network is also much more reliable than a cellular network that the agency has no control over. It's the same reason agencies maintain their own data networks, rather than relying on commercial services to carry their data. During the Northeast blackout of 2003, cell phones were rendered practically useless. Public safety radio networks kept on chugging along. Even if the repeater sites fail, there's almost always a backup simplex transmitter in place. At the very least, the radios themselves can usually fall back to some form of simplex point-to-point operation, so you're never completely out of touch with someone.