Atom hasn't been discontinued; low-end "Celeron" and "Pentium" chips are actually Atoms.
Both Pentium and Celeron are used for standard mainstream dual-core chips and for Atom chips (eg. J4205, Celeron G3900, Pentium G4400)... so in a way they are Intel's trash-can brand.
I think you meant the mainline's efficiency increased. :)