GF has been trying to save Dex space while still adding new designs since 6th gen, and the variants are the new way they found to do it.
Except we don't know that. It is one of the theories that people have come up with as to why there is less new pokemon, but by no means the only theory or the only probable theory. Just like we have no evidence at all that the Alolan forms have "taken away space" from new pokemon or megas (or that megas have "taken away space" from new pokemon)
Now I agree that some of the Alolan forms (Ninetales, Exeggutor, Marowak and Sandslash) are more distinct from their original forms and more creative than the Pokemon you named. But it's not like the advent of Megas and Alolan forms (or the smaller dexes) suddenly meant that there were no less inspired pokemon anymore.