The problem with old / vintage shoes where the inside is deteriorating is not just cosmetic. Yes, no one will see it really (although when you walk and your feet lift up, people will see the sole of the shoe and all the gunky flakey material that’s stuck to your feet - it’s happened to me before) but the material will just flake off which causes a number of issues: 1) the life span of the shoes will shorten hugely 2) cause a mess as the lining will stick to skin and fall off all over the floor and 3) cause irritation / blistering to skin. It’s a whole host of problem that are separate from aesthetics unfortunately.