I get your point, but I disagree on the part where you write that background operations need the disk or else they can't possibly work at all. Current smartphones are not designed to work without accessing the disk, that's true, but in theory 1GB of RAM is plenty for processes like polling a chat server or SMS to run entirely in it.
Yes, but if the phone was to lose power, like e.g. run out of battery or crash you'd lose all the data in RAM. From a general usability-standpoint losing any data that does only reside on your phone is unacceptable, most people would definitely throw a major fit about it even if it only ever actually harmed very few individuals. What if that SMS-message you just got, for example, was something important, but it got lost in bit-space? Well, you wouldn't know until it's too late!
As for keeping things in RAM: Android isn't known for being small and efficient, not even Lollipop. There are plenty of RAM-hungry background-tasks running and things like e.g. Facebook and such that are notoriously bad. Then some phones even offer things like spoken keyword-recognition that would require keeping a whole effing lot of stuff at all times in RAM just to work. I doubt 1GB RAM would be even nearly enough on most people's phones, though 2GB or more would of course help. You'd still have the issue I mentioned above, though.