There's nothing wrong with planning for a certain device lifespan, provided it's long enough. But there's a bell curve here: it doesn't make sense to plan for 20 years of support if 99% of the devices are broken or replaced by their users by then.

Also, Apple repair prices are high, but not outrageous. There will always be someone claiming they can do it for less, but not many that will give the same guarantees Apple does.