When I thru-hiked the Pacific Crest Trail, I had to ship boxes of food and supplies to locations along the entire 2650 mile length. Sometimes it would take two days to leave the trail, walk to a trailhead, hitch a ride to the town where the post office was holding the package, and then return.
All of that meant that I needed to plan for replacement shoes before they 'blew out' of their useful life. No shopping along the way where I could easily depend on getting the shoes I wanted. I decided the best course was to plan on a shoe replacement - whether needed or not - between 500 and 600 miles.
So I bought 5 pairs of identical shoes to ship with my other supplies in some of the boxes for pickup. Sometimes the replacements were needed, sometimes I could have gone farther along the PCT with what I already was wearing.
So if you can, get the Cascadia version that you like, and replace the old ones. (good shoe, btw). I will typically allow no more than 800 miles, even when not backpacking, due to my unique needs. But, I will use the old ones that I replaced as around-the-house chore and wear shoes.
A lot of times, the midsole cushioning has broken down before 900 miles of wear. Trail and street running shoes are designed to do that. The impact cushioning seems more effective with the type of materials in 'cushioning' that create a sacrificial deterioration.
In fact, the uppers and even the outersole can look pretty good even with that much mileage when the midsole components are not doing the same job of absorbing impacts.
A good, third party insole can help extend the life of the shoe by taking up the role of help to absorb impacts, but that is only a temporary solution at best. Even then, a lot depends on whether the other components of the shoe are still functional - - the motion control and stability stuff. If the shoe shows that is is rolling to the inside or outside (pronation or supination), then it can be damaging to the ankle and foot.
Sorry for being so wordy, but I hope that gives you some ideas. Send me a PM if you think I can be of further help, or I'm happy to post answers on the thread, too.