1. Why should non upgradeable RAM reduce the useable lifespan of a computer?
Hmm. My mother has a late 2009 mac mini, which she uses mostly for e-mail, browsing, and photos. Never had a problem with it, until earlier this year, it suddenly slowed to a crawl. I took a look at it, and the thing was completely out of RAM; even with no apps running, it barely had any free RAM left. The problem turned out to be that she had upgraded the machine to OSX 10.10, and the default amount of RAM that came with the machine (2 GB) just wasn't enough to run both the OS and an application at the same time.
So, I upgraded her machine to 4 GB, and she's a happy camper again.
I guess the thing here is, what seems to be plenty of RAM now might not be years down the road. That 2009 Mini is still perfect for her needs; but if its RAM couldn't be upgraded, it would have needed to be completely replaced. (That, or downgrade the OS and try to avoid any future updates...)