My like for Weird Al aside (I wonder how many people remember he actually did a song for Pokémon), Pokémon games are a fascinating subject. Mainly because there's a lot a misconceptions about the focus of the games.
So here goes: Pokémon are first and foremost a series of multiplayer-focused games. The single-player story is primarily a gateway/tutorial to get people into the multiplayer.
While I do know that some people do find single-player playthroughs enjoyable - even me, as I'm working my way through my copy of SoulSilver - the truth of the matter is that the primary appeal of a solitary run through these games is not emphasized by the game design.
There are two factors that tie into this. First, each game usually requires you to assemble a working team of multiple mons before a certain point in each game, or otherwise it would be too difficult to progress. Logically, it follows that any Pokémon that can only be obtained after this point aren't really viable for single-player. You get them too late to actually be able to use them.
The second is the idea of variant Pokédexes versus the national version. Each time, an increasingly large number of the pocket critters are unavailable to you, usually until you beat the game. Again, that's too late to use, so you're stuck with a smaller set of mons.
This may not seem like a problem, but the entire appeal of repeat playthroughs of the games is building different teams and seeing what works. Which get crippled when you realize there's not actually that much variety as you would think. Discounting unevolved ones, your typical game gives you a selection of somewhere between 60 and 90 to use. Which, while not bad, isn't exactly the greatest variety, especially compared to how many possible team combos there could be.
This explains why the games have been steadily made easier and easier. Less dependence on HMs, permanent TMs, reduced gym leader and Elite Four difficulty, the Experience Share buff - it's a long list of changes that all point towards getting players through the single-player faster to get to the post-game, and the multiplayer that comes with it.
Sun & Moon may shake up the formula somewhat, but without evidence they are front-loading the available Pokémon, along with making a much larger variety available from the get-go, they're not addressing the main issue. Nintendo is still making these games multiplayer-first, and will likely to continue to do so.
Not that it stops you from "catching them all", but be aware that if you bought any of these games to play by yourself, you may be disappointed. They aren't bad, but there's a lot of wasted potential there. It's a bit of a shame, really. I suppose at least I can enjoy raising my Bayleef. (What? I like Chikorita better than Totodile, and I was using a different Fire-type anyway.)
Smell ya later! (Gary/Blue's jingle plays as I walk off.)