That makes one of us. XD
I have heard this before, but it mostly just nerfs a few of the older decks... or am I missing something?
Here are my feeling on this.
To get an idea of how rotation can affect the current state of the game, let's look at what (somewhat) relevant cards we lose based on what I believe are the 3 most likely rotations:
In NXD-on, we lose:
Eelectrik NVI
Cobalion NVI
Energy Switch BLW
Energy Retrieval BLW
Super Scoop Up BLW
Gothitelle EPO
Crushing Hammer EPO
Victini NVI (Victory Star)
Kyurem NVI
In DEX-on, we lose (in addition to the ones listed above):
Mewtwo EX NXD
Shaymin EX NXD
Zekrom EX NXD
Level Ball NXD
Heavy Ball NXD
Double Colorless Energy NXD
Skyarrow Bridge NXD
Prism Energy NXD
Zoroark BLW
Emboar BLW
And in DRX-on, we lose (again, in addition to the above):
Empoleon DEX
Raikou EX DEX
Sableye DEX
Darkrai EX DEX
Tornadus EX DEX
Zoroark DEX
Dark Claw DEX
Dark Patch DEX
Enhanced Hammer DEX
N DEX (Maybe, depending on future reprint possibilities)
Pokemon Catcher EPO
Disturbingly, we lose very little even from the deepest possible rotation, but things do change a little bit with each possible rotation.
NXD-on would affect the current meta very, very minimally. Only 3 top tier decks at the moment would be affected by this. Eels decks go away entirely, Gothitelle/Mew EX/Accelgor goes away entirely, and Darkrai loses Energy Switch and Crushing Hammer (which is actually a pretty big deal). We lose 2 decks with this rotation and one would be slightly worse, but with the addition of a new set, we would actually likely have MORE viable decks, even with the 2 losses.
DEX-on changes things a bit more, In addition to my previous comments, we lose Mewtwo EX who has been dominant or at least very good the entire time it has existed. We also lose a few EX that have been relevant, but aren't really anymore. We would lose DCE and Prism, which are essential to many decks, and we also lose Level and Heavy Ball as well as Skyarrow Bridge, all of which have and still are used in many competitive decks. Many decks would change due to this rotation, but very few would actually disappear. Mewtwo-centric decks would go away, but most decks simply use Mewtwo EX as an alternative attacker, so they would find other attackers to use. The loss of level ball would slow decks down slightly, and the loss of DCE and Prism would require some decks (such as Plasma) to completely restructure their energy lines. Still, while the way decks are built would change a bit, the metagame would remain largely the same.
DRX-on changes things a lot more. We lose Darkrai EX, a card that have been either dominants or very, very good it's entire lifespan, as well as the entire Dark engine (Sableye, Dark Patch, Dark Claw). We lose Tornadus EX as well, another very good EX. We would lose Enhanced Hammer, making Special Energy far less risky. We would also probably lose N, which is a MAJOR part of the way the game is played. Every deck has to use at least 3 copies, probably 4, and every players MUST take the cards existance into account when building and playing decks. And then we also have Catcher, which is also a very big deal. Your bench now becomes safe, making bench sitters MUCH better. Super fast decks like Plasma are also hurt a lot by the lack of Catcher, since taking easy, cheap prizes is much more difficult. With this rotation, we'd lose Darkrai EX decks entirely, we'd lose all Dark-type support, we'd lose fast, aggressive decks that can't hit hard, and we'd lose a powerful comeback and disruption card.
If it isn't obvious already, it seems like only DEX or DRX-on would change the game significantly, and DRX-on would change it the most. I think, in particular, losing N and Catcher would fundamentally alter the way the game is played.