define "life span"? a laptop only "used by four-year-old" is pretty dead laptop. can it fully replace your every-day development/whatever machine?
also less repairable/upgradable device is often more solid and reliable.
dropped repairability is just a tradeoff between delivering more "awesome" device and delivering longer-lasting device - no wonder apple has taken the path it did.
p.s. no-one forces you to throw away laptop you own just because new version was released. in fact most macbook owners will just sell current devices and in 10 years time it might still end up in hands of 4 year old
Now I haven't actually upgraded it in that time, so it's not really a data point against unserviceable machines but it's sure as hell a data point against 2 year life spans.