Switch over to what, exactly? And what if they decide to switch to SaiaNet, then those "cheap dmr radios" all of a sudden become relevant. Believe me, there are much cheaper and less capable radios out there than the XPR series. APX are nice radios, but are very expensive. If there's not a lot of P25 in Erie County (which there isn't), they they are not being used to a fraction of their capability. DMR/TRBO is popular and growing more popular, however.
I have no idea why APX vs XPR for the Cheektowaga Depts.
I can tell you that Cheektowaga FDs and PD are NOT going to any form of digital modulation anytime in the 5 years at a minimum. They spent ALOT of money on their new simulcast analog system.
Possibly interop ? Not every FF would need to talk on P25 to another agency but maybe some do ? Its really no different than a FIRE department having some trunking radio and some conventional only. No need for everyone to talk on trunking. Lots of reasons an agency would use some or any of the above.
Radios are often purchased with grant money, and federal/NYS grants require radios to be P25 capable (they don't need to have P25 enabled, just be capable of the protocol). So if the agency is spending their own money, they would likely gravitate towards the cheaper XPR option, and if they're getting "free" radios through a grant, would have to go with the APX or other P25 capable radios (Kenwood, EFJ, Harris, etc.)