A lot of people do resource-intensive tasks on a Mac, much more than you may think. Indeed, a lot of people buy a Mac over a Windows machine primarily for video-editing, running virtual machines and graphic design. Mac consumers don't want to pay the ridiculous prices that Apple charge for an extra 4GB of memory or 250GB of storage space when the parts itself cost a fraction of the price they are charging. The option to have user-upgradeability is something which extends the life of a product and gives consumers the peace of mind that they won't have to shell out for a whole new machine should their needs change or make a decision now as to how they see themselves using the product in a few months/years time.
And like you said, should the RAM fail, the whole logic board will need replacing and that isn't cheap, especially if you're out of warranty.