For anti virusing etc after the cut-off, it would be sensible to use one of the non-MS offerings. But whether paid or not, they will only carry on for so long, so the day of doom will approach anyway. Buying a new OS with built in security might cost less than subscribing to a security outfit.
Another factor is the actual PCs hardware. It is quite possible that something in or around there will pop its clogs, and it could well be that a backward compatible part is no longer available at a sensible price. A new box with the current OS might be the economical answer. Also, a lot of new software relies on features in Windows to work. If its intended for the current system, it might well not work on an older one.