partly it's not about what it can do that a smartphone can't, but exactly the opposite. there are lots, and lots, and lots of things smartphones can do that PDAs can't. and i neither want to do any of them, nor have constant notifications and advertisements trying to get me to do them.

but the big thing for me is that they were designed local-first. my data lives on the device. i sync it to my other devices. i can't accidentally lose access, get suspended, have my account closed due to a change in ToS, get my account locked out because i accidentally visited from a VPN or used a VoIP number -- all of which, now, are at best massive inconveniences and at worst result in major data loss for no reason. it didn't need connectivity to do it's job.

when i can refuse to connect my smartphone to an IP network and still have it do everything i need it to do, i will back off :p

i have an android phone here that i've slowly been trying to make behave more and more that way, looking to tools like SyncThing and DECSync and anything that's designed similar. it's close, but it still causes enough annoyances that i'd rather just have a PDA.