We're talking about Linux users here, not your run-of-the-mill generic end user. If you download an AppImage, you're most likely an advanced computer user. Maybe you don't download media yourself, but you'll probably have huge node_modules/cargo/venv/maven directories slowly clogging up your drive.

AppImages aren't easy to use (you can't double click them like on Windows and they all have to provide a mechanisms of their own to register a shortcut in the system menu), their startup time is affected by the compression tying all the files together, the security is no better than any other application (worse, in fact, because when I update my system's openssl client I'll still need to wait for every AppImage program to publish a security update with the patches included, which usually takes months or longer). I don't know about RAM usage, but the duplicate libraries being loaded by the executables will cause at least a few megabytes of unnecessary RAM usage.

AppImage is mostly there to help the developer spread the software. The benefit for the user is "at least there's something I can run, I guess".