Whew, divisive comment thread. Ian Sane (and the article if anyone read it) already brought up the important point that the "mainline series" game comes in 2019, so I'll only touch on this announcement.
This game is great for someone like me who generally has had a falling-out of the series. Like many, I had a copy of Blue on Gameboy and had a neighbor friend with Red who I battled with and traded to get all 151 Pokemon (+Missingno). I didn't pick back up the series until X/Y after seeing if it would capture me and my wife's interest if we played together, but we both promptly dropped-off. The game felt bigger in a way I wasn't sure was good for it, the Pokemon total count felt daunting, and the battle system largely felt the same as when I played Blue to a point where it felt stale and boring.
Kanto is a great place for it to be located for me, because it'll have a familiarity and comfort zone if I decide to give the game a try. My son watched the first cartoon first, too, so he'll love that Pikachu can be his starter Pokemon.
It being the Original 151 is great for us because of that same familiarity, and it's a number that's a little more manageable than the number in X/Y (450+?!?). I'm not at all opposed to them adding/amending new Pokemon as the series goes along, but for me personally, 250+ is where it gets burdensome to even attempt catching them all.
Most importantly, and maybe it's something not everyone can relate with here, this is exactly the kind of Pokemon game I could share with my son. He can't read yet so he can't play a traditional game, but he definitely remembers most of the Pokemon from the shows, and loves the concept of Pokemon go where you find them out and about and catch them outside. He's going to love swinging a joycon at the tv to throw the Pokeball to catch them, and having a game that we can play together in a series he loves.
If this hits the right notes, I could even consider giving the mainline series a chance if it re-ignites my interest in the franchise.
I can see if you die-hards are disappointed the mainline series isn't coming til next year, but please try not to hate just because it exists.