I am looking at the big picture. I do agree with multiverse theory with regards to anime, games, and manga, they are different enough to warrant such an explanation. The whole "why do we never hear of Unova and Kalos" could be because the first four regions are so close together and the other two are isolated from the others and each other (based on real world geography). The first four regions are close enough to form a state really easily, and enough of the action happens there to be oblivious of Unova and Kalos, and only pay attention enough to them when something does happen. It is like the USA, which is arguably the leading power in our world, but ask the typical American where Armenia, Serbia, or Rwanda is, most can't answer nor do they care. They also weren't talked of before they each had their genocide, which corresponds to stories in Unova and Kalos, both of which happened after the first four. It could also account for why the first four are mentioned in the latter two. The Professor Oak example wasn't from the games, so it is dubious to begin with and doesn't count, since it is a different established universe. The dimensions thing is a little different. Dimensions aren't the same thing as universes. Both universes, by the logic proposed before, have four dimensions to them, meaning Platinum's dimensions aren't necessarily evidence.
I will agree with that the timeline is a mess, and the multiverse within the games does explain it easily, but it isn't the only way. Time could be structured with "fixed points", points in history that cannot be changed, with the rest of the timeline in a constant state of flux, all the while remaining in the same reality. With Dialga and Palkia regulating time and space, respectively, and with most of the player's choice being up to the player and not determined by the game, this could work. As for the connectivity between XY and ORAS, time travel could still work here, as shown by the connectivity between the first and second generations.
It is a lazy plot device simply because it doesn't require much and apparently fixes everything.