How many upgrades does average consumer make on their MacBook Pro? I'd argue it's ~0.
When I worked at Geeksquad we used to get people in all the time for RAM upgrades ($29 labor charge).
I'm still using an early 2008 macbook pro thanks to a cheap RAM upgrade.
Additionally there is a huge secondary market for macbook pros. Plenty of people help pay for their new $2k+ computer by selling their old one. That won't work if it's completely obsolete in 3 years.
My manager just "upgraded" his old Mac by installing an SSD and gave it to his wife to use. Said it prolonged the life for another few years.
I just upgraded RAM and HDD -> SSD on a friends computer. He had no clue what I was doing, but his computer ran "better than new". All he does it surf the web, use Outlook for email, write Word/Excel docs. Extended life for another few years for him. He certainly doesn't need a new computer (which is was about to go buy). If users themselves can't or don't know how to upgrade, they may well know someone who can. Or some small local indie shop who has the skills.
Upgrading memory is especially attractive, since it's so much cheaper to buy it practically anywhere else then Apple.