I have declining vision (as you mentioned of yourself) and my advice is to purchase a good/better unit now (keep the old functioning one as a backup).
DeKay
Should I buy another flat screen now while they are still relatively cheap?
My current Vizio 65" is either 7 or 8 years old, and it gets a lot of use. Is there an average life span on these? My previous Samsung 70" made it about ten years as I remember. I see that WalMart still has flat screens in that (or around that) size for relatively cheap.