Alright, I figured out the reason why getting a smaller amount of Pokémon this gen bugs me. Normally I'm fine with whatever number it is, since I like the designs, and that's what's ultimately important. But looking over all the new designs, the reason finally hit me:

Every gen, we get 3 starters and their evos, a regional bird, regional rodent, a bug line, and some legendaries. This did not change in gens 6 or 7.

Gen 6 gave us 72 new Pokémon, but those 9 starters plus Talonflame and Diggersby and Vivillon are already 17; they had the foresight to only add 5 legendaries and mythicals, but that's already 30% of them!

And now Gen 7 is giving us (presumably) 73 new Pokémon, and it's the same: Starters+Pikipek+Yungoos+Grubbin is again 17; I don't know if the Tapus are legendary, but we've got 4 already in Solgaleo, Lunala, Magearna and Marshadow. In fact I'd assume the Tapus are not legendary for this reason, since it gives pretty much the same ratio otherwise.

While I understand Megas and UBs and regional variants will add more variety, and I'm fine with that, I'm increasingly worried that all the new Pokémon species we get will follow this pattern, causing a larger and larger portion of the Pokédex to be these kinds of Pokémon.

Am I worrying about nothing or do you guys share my concern?