Ever managed to upgrade a bad shoe?
I routinely switch out the insoles in my new shoes. It won't make a bad shoe a good shoe, but it can make a shoe that doesn't fit comfortably or that is causing injuries due to how your foot fits in the shoe. Sometimes, but not always, switching out old inserts can add a few more serviceable miles to an old shoe.
I have at least four shoeboxes filled with old insoles and aftermarket insoles, including several Superfeet off-the-shelf orthotics and dozens of orthotics I have made myself from old insoles.
Keep in mind that insoles are made to fit the average person who is likely to buy that shoe, but few people have perfectly average feet.
In fact, feet are like snowflakes. No two are exactly alike. Even your left foot is not a mirror image of your right foot.
It's kind of like buying an off-the-rack suit. Most buyers are going to have to get it tailored to fit right. It's the same with running shoes.
Got these flats I hate the feeling of. Too firm for my liking. Went to a cheap-store and bought some gel insoles for 3 dollars. Inserted them into those shoes and ran 15 km yesterday. Usually I hate life when finished running 10 k but now I would gladly run 20 k with them. Turns out I just got a new pair of daily trainer, inbetween a plush premium and flats. Best 3 dollars I've ever spent! Changed my feelings about the shoes 180 degrees.