Why buy one? Just because I can. I have a thing for SW rimfires and seem to collect/accumulate multiple examples of them. From a pure user/practical point of view, the OP is 100% correct. His 18 has served him well and why get rid of proven woodland companion. I wouldn't. I feel like that when it comes to my other firearms endeavors. Take my quail gun for example. This early production 20 gauge Browning Citori Superlght (pre interchangeable choke) I bought new in the 70's has been through thousands of both miles and birds. Despite being oiled at the end of each hunting day, there is not one iota of bluing left on the lower receiver and the stock is dented and worn. It is beautiful! No way I would trade it for a new, shiny one.
For some strange reason, I can't make myself put that kind of wear on my beautiful, vintage pristine blued Smith rimfires (17,18,34,51,etc) when stainless options are also available in the same formats for my hunting and field carry needs. So I have also ended up with a 63, a 651, a 617, etc. through the years. My blued ones have remained relatively prestine other than worn ejector rod bluing from shooting them a LOT on my home range.
There is no right or wrong here. We all spend our resources differently and our priorities/interests differ. BTW, my new 617 Mountain Gun showed up at my dealer yesterday for a pickup this afternoon. I have feeling the 18 will get shot as much as ever and the Mountain Gun will get holstered and carried a lot afield.