Are they alternatives? Yes.
For a large number of average usage scenarios, it suffices. If you need x, you'll likely know that you need x and continue to use x. For the average Joe who uses "Microsoft Word" as a synonym to describe "word processing", or "Photoshop" as a synonym to describe "image editing", it'll be fine. The same people call game consoles "a Nintendo" or "a PlayStation", even if it's an Xbox, same people who called any generic MP3 a player "an iPod". That's fine.
This same train of thought are the people that parrot "desktop Linux can do everything now". For the average Joe, perhaps. Need to use a niche piece of hardware or want to install Linux on a modern-day laptop and/or use every piece of hardware on your device like a docking connector on a ThinkPad? Forget it. It's half-baked for the power user that cares about niche features.
Maybe there are some Photoshop features that are not implemented in Gimp, but hey we don't need those anyway.