The big issue is when older communication standards stop being supported and you lose access to 3G, 802.11n, TLS1.2, etc. at that point, the device becomes an offline-only device.

That said, I have an iPad 2 I picked up in 2011 that still functions perfectly except for web browsing. Works great as a jailbroken emulator, an eBook reader, running all the same offline apps I used in 2011, and the original battery still lasts for 8 hours on a charge.