I've still got my old Psions...... broken ribbons and failed backlights were the problems.
To me the beauty was the independent platform, outside Apple and Microsoft and I could do everything - take notes, use address book, check diary, form documents etc. etc.
The other big plus was the ability to draw on screen - you still need separate apps to do that on a smartphone.
What it always lacked was the connectivity so you couldn't just go online like a smartphone and surf the net/web. Battery life would be crucial to that now.
New tech could so easily solve that.
Can I swap an old one for a new one...........? ;-)