If you purchase 6.14, which has no future updates or support, you do own it forever, but at some future operating system update, it will stop working, and then you have nothing. Part of the value of subscribing to Lightroom Classic CC is that it will not become obsolete and useless because of operating system changes.
You don't really "own" Lightroom 6 .14. You only have a license to use it. You'll have to decide what is of value to you. With no more updates, if you purchase a new camera in the future that isn't supported as far as the raw files are concerned then you will have to convert the images to DNG using the standalone DNG converter (which is free from Adobe) in order to use your raw files. Raw processing technology will continue to improve, and you won't get any of those improvements. So that decision is yours. The $9.99/month plan also includes Photoshop. I don't know, maybe that isn't of any value to you. It's a nice program to have when/if you need it.