Powerful cards

This game is not based on "1 OP card wins the game". There isn't a single card that would win the game on it's own or with several copies of itself. Most of them also have the proper cost/risk/reward to keep the game interesting.  Cards that tend to defy this rule are mostly the legendaries and some event cards, but this is only on a first look. Often they are just as easily countered or overpowered by the other side as players try to win against a new meta.

I do agree a game can suffer somewhat of new cards that get stronger compared to the previous sets. This makes it so that older cards quickly become more or completely irrelevant. Example: i hardly ever see someone playing the Repeater pea, and favor Cosmic Pea more because it also conjures another card with double strike in trade for -1/-1. You normally play this card with a torchwood or podfather so the 1 damage you lose per strike is less relevant compared to the card you get. I understand a lot of cards are just fodder, but rare cards from set 1 became obsolete as new cards became available to replace them (from either event or set 2)

New sets also bring new features. Like the galactic set brought overshoot, enviroments and Conjuring (on a grand scale). This again makes older cards infavorable unless they work together with the new cards (and not the other way around in most cases). In MMORPG terms, this is similar to the gearing itterations where previous armors become obsolete in most cases.

Does this hurt the game of itself? It doesn't take away from the strategy part. Making new cards is essential to keeping the game alive. if no cards are added, the game would become stale. If a card seems OP, then probably the playerbase has yet to find a way to counter said card, since there is already a lot to go around with to fight any deck. 

I guess at a certain point it's inevitable that some cards will become irrelevant. 

Hearthstone and MTG both have different formats for that exact reason. To protect new players from the Incredible high collection and/or Buy-in efforts they would have to make and make sure they buy the newer cards anyway. That also prevents these games from the YGO-system where every set has to be essentially straightup-stronger or has to have idiotic new fusion rules and mechanics in order to keep the game alive.
If you offer formats you gotta have an "open" format and try to keep a steady powerlevel for newer sets. With that concept players of the open format would need to have a large amount of strong cards at their disposal but would have to get only hold of a few cards of each new set, the best of each set so to speak, while players of the small format would potentially have to get everything of each new set but don´t have to concern themselves with buying old, potentially expensive cards.
Hearthstone has a "classic" set for this purpose which is a collection of cards that are available for both formats at any given time while magic has several formats and much bigger sets.
One could say that MTG is abusive in that regard but you can technically just buy exactly what you want instead of ripping packs and wait for enough dust/sparks or the lucky drop to get what you need.

The downside of a format system is that you basicly see the same card over and over again with just minor tweaks. but to be honest thats better than just having stronger cards with every expansion. 

look, you guys are giving the indication that you are Trolls who are terrible, at the game, it is not about only having powerful cards, it is about using cards the right way, see I am always taken out because I sometimes try to think offensively, and lose before round 5 is even finished. that is a problem with a lot of younger players, they are thinking bigger is better. it isn't, it is about balance. 

I would agree with you if this game actually had good removal. Removal in PvZ:H is sort of expensive and/or highly conditional.  The only reason this is remotely balanced is because dmg dealt to Zombies/Plants sticks around so you can kill them in multiple tries. 

The thing about using your cards right is also a thing of cards your opponent can answer with. Right now you can just play your stuff whenever you want. Especially if its above the 4/4 mark things get incredible hard to remove without using almost all your ressources. I think its pretty unfun that removal is oftentimes more expensive than the card it removes....it should be the other way around.

P.S.: I see how good removal is frustrating for new players but everone experienced with TCG´s should now that things like locust swarm could very well cost 3 or 4 and be perfectly fine anyway