First, the Discworld reference is wonderful! Second, I hate to say it, but you probably need two pairs of shoes/boots. They collectively can last the rest of your life, but shoes tend to do a lot better if you can let them fully dry out (even just from the sweat of your feet) for a day between wears. If you’re wearing the shoes in bad weather, this is even more important.

I would advise getting the new (or new to you) shoes while your current ones still have some life so you can alternate. Then get a second pair of BIFL shoes when you’ve had a chance to save up again. By that point, you’ll also know if the good shoes you got hit all the marks or if you need your alternate shoes to have slightly different qualities, e.g. better water repelling in the new shoes but you can sacrifice a little bit of looks to get it.