Ease of use, RAM usage, startup time and security should all rank higher than disk space.
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".
Uh, but that's exactly how they work. What file manager are you using them that doesn't recognize them? Also you can just chmod+x them and execute them.
As for their size, my XPS13 has a 250GB SSD too, not easily upgradable, but AppImages just aren't a significant burden to me. Once a month or so I move downloaded media to my NAS, but all of my AppImage applications together add up to maybe 15GB or so, the redundancy in them is really the least of my concerns.
Dolphin does seem to support AppImages natively and will execute the file (after clicking through two security prompts). I guess KDE users are AppImage's target audience, then?
There's a whole range of tiny/small computers that run Linux and that absolutely don't have hundreds of GBs of storage space.
