That's some long-term support. If your iPhone doesn't get OS updates, it almost instantly turns into a brick. Android doesn't get updates for nearly as long, but you won't lose apps and stuff nearly as fast.
Sure, with custom ROMs you can carry phones much farther than intended, if you are lucky enough that your phone gets updated custom ROMs.
But I wouldn't equate 3rd-party support with 1st-party support.
Also, custom ROMs that provide newer Android versions than the original 1st-party support often don't upgrade everything, e.g. the kernel is usually not upgraded, which can cause issues in some circumstances.
But yeah, it's pretty great that there are people who spend their spare time to get newer versions of Android on old phones. Should be standard, that phone manufacturers are doing this themselves, sadly it's not.
Heh. Well that's interesting. I'm using an old mobile for my VOIP. Happens to be KitKat OS. Was able to extend it's life with a replacement battery off ebay, but VOIP softwares were starting to all update beyond the limits of my device anyway.