That was how it was branded—Paint 3D replaced mspaint.exe and it was positioned as an upgrade (Paint → Paint 3D). But there was substantial backlash when Paint 3D didn’t do all that mspaint.exe had done, and Microsoft caved to pressure and put mspaint.exe back a build or two later.
Hardly an "upgrade" if functionality that people use gets removed...
Something can still be an upgrade for the vast majority even if it doesn't do 100% of the same stuff as it's predecessor...
to be fair, how hard would it be to have feature parity with an app as simple as MSpaint?
You wouldn't think that hard, but look at this thread and all the different ways people are using it. And for them not having feature parity could mean changing keybindings!
Plus it does do some off the wall stuff, someone in this thread mentioned it can pull images from a scanner!?