Yeah, you take your deck and adapt it. Take out the old, bring in new. If you have a deck that you havent modified since the last rotation, chances are you will have to start from scratch the next rotation.
I think that's a good way to keep the game fresh and continue selling. Because the incentive to buy boosters would diminish if you were able to play any card you want. Let alone how broken it would be.
The creators see first hand in competitions what they like and what they dont. Exploits dont get re-printed. Balanced cards find their way into mainstream formats and get reprinted every 2 years or so.
I just started buying again and was never into the competitive part of tcg so I bought quite a few hg/ss booster packs (for gengar prime) and later realized I had maybe half a year before they couldn't go into a modified deck. =( sad, but you move on and now im looking forward to a Mewtwo ex deck. Goodluck with that and you can always play unlimited if you're short on cash.