I guess it's not a mandate for Pokemon to release a card for every Pokemon, but leaving variations out when there was ample time and space to include them rubs me the wrong way.
Was there a need for another Mimikyu? There were also 2 Tapu Kokos.
If we're talking replacing type for type, then for Steel there was Scizor, Aegislash and Metagross. 2 of those are popular mons, but still. Psychic had Gardevoir, Sylveon, Espeon and Mew. I guess no spaces outside of popular mons. Fighting had Stonjourner and Lycanroc. Oh, and I guess you could mention Appletun, but no one cares because GF was too lazy to even create a separate form for that.
But is there really some sort of game balance that needs to be taken into account? Don't play the game so I don't know. But obviously they've done this before with the various Megas being left out.