Hmm, not sure about that. As I've mentioned in another post, my mother purchased a 2009 Mac Mini with 2GB of RAM way back in 2009, and has been using it for mostly web browsing, e-mail, and photos, tasks which it performed perfectly for her. That is, perfectly until earlier this year, when it suddenly started choking on those same apps. It turns out that she had upgraded the machine to OSX 10.10, and it no longer had enough room to run both the OS and her apps at the same time. Upgrading the RAM to 4GB solved that issue. If her machine was not upgradable, it would probably have needed to be replaced (or, I guess, downgrade the OS and try to avoid any future updates...).
Also, the ability to upgrade RAM gives a machine flexibility to be used in different roles as time goes by, where machines with soldered RAM might need to be thrown away and replaced with something new...