Of course, Riolu wasn't in the Unova Dex before BW2, either, I don't think. So they could've just added the Machop line in instead of the Riolu line if they so chose.
There's nothing wrong with having one more Pokémon in the regional dex. Nothing says that the number of Pokémon in the regional dex had to be fixed.The Machop line has 1 more Pokemon than the Riolu line, so if they wanted to add it in, they'd have to remove something else.
But if they did remove a Pokémon for Machop's sake, they could've removed Lapras or something.
Besides Starmie, Azumarill's basically your best choice for a Water type if you didn't pick Oshawott. It has an advantage over STarmie in that it comes way earlier, even if you opt for Route 6 Marill over the absolute bullcrap that is Azurill.
Why not just remove something post-game, like the Slakoth line?