so why on earth would I just buy a liner?
You've been happy with the fit and performance of your current boots and don't want to go through the process of finding new great fitting boots again. Some people have funky foot anatomy, so finding great fitting boots is difficult. So, for $200 (or less) you've doubled the lifespan of your boots.
Also, often times replacement liners are of much better quality than stock liners.
I've never gone this route, but if I found some boots I really liked, I'd consider it.