I’m going to leave my thoughts about why I think rotating sets in a growing CCG are necessary, since it seems like a good place to make the argument.
If the card pool in a game with a ladder/ranking system is ever growing, your design space is shrinking. This isn’t just because you have less new card possibilities (“Oh, this new card is nearly the exact same as one from 3 sets ago but with better stats!”), but also because every new card created has to be weighted against every other card that has already been made. This makes is very hard to balance some cards as they are made. Alone, a card the community considers good or even somewhat powerful but not broken can become broken when it interacts with another card in the game. Think of the infamous Aspect of Shimzar and Thunderhorn combo. Both cards are actually good, and many would even argue that Thunderhorn is overpowered, but there are a lot of people who do not mind these cards individually but HATE how strong this combo is.
Now imagine that every time a new card is released there’s the possibility of another card combo like this coming to the surface. It can be predicted and avoided in the design stage, but the normal way to do this is to remove the function of the new card that made it unique in the first place. As such, the design of the card had to be changed in a negative way just to promote healthy, but boring balance.
There’s also one more reason to do set rotations. Yes, they DO want to design and release newer, powerful cards. They want to do this to make the game more fun but without trivializing deck building. But if the card pool becomes big enough, eventually deck building will become flat and boring. You just pick all of the best cards you can run and maybe fill in a few optional slots for other purposes.
Furthermore, you can expect they want to incentivize players to buy or earn packs from their new set. The way to do this is making strong, appealing or more interesting cards. It’s very hard to do this without ruining the balance of your game if you’re always adding powerful cards every expansion. The game can and will break. It’s not about how good they are at balancing cards, it’s a matter of providing players options. Adding options, itself is powercreep.
To those of you out there who are nay-sayers. I want you to understand, I’m iffy about these changes too. I don’t even like to play competitively. But I also want you to see why this is necessary for a healthy game. There may be other options out there that can be taken, but I don’t know them and I don’t think any of them are proven yet.