I would be very surprised if many frameworks are upgraded ship of Theseus style for decades, or if the total cost of ownership (and even ecological impact, most of the nastiness is going to be the electronic internals, not the metal casing) is lower than for someone buying a more integrated laptop ever 5-6 years.
Hard to say. If people boast about a ThinkPad lasting a decade, I see no reason (post Moores law) that this can't last that long. The only think not obvious on how to replace is the screen and speakers.
A good test would be to work out what's the oldest in-use Framework (which should be one of the first, if not there's an build quality issue) and see how many upgrades were needed to keep it functioning compared with similar era machines from other manufacturers.