Let's take Hearthstone for example. In Hearthstone, if you want a card, you spend money to open booster packs, and eventually you'll get whatever card it is that you wanted. But if you don't have money (or aren't willing to spend money on micro transactions, which is perfectly acceptable), then you could just build up in game gold to buy boosters. If the boosters that you opened didn't have anything good in them, then you could turn the unwanted cards into dust. Once converted, that dust could then be used to craft the card that you want. So, there wasn't a single card which was exclusive to P2W players. Literally everyone could get any card they wanted, as long as they were able to put in enough time or money.
Deck Heroes, on the other hand, does not have such a luxury. If I want an Aquarius, but don't have money to buy gems for boosters, then I'm shit out of luck. That wouldn't be such a big problem if the P2W exclusive cards weren't so game changing. Creatures like Taurus, Aquarius, Octavius, Kitsune, and Great Mystic can prove to be a real advantage when used against someone who doesn't have them. Unfortunately, F2P players like myself will likely never get their hands on those creatures, unless they get really lucky.
This is why the Mines were such a godsend. It made really rare creatures and heroes like Mythril Drone, Oinari, Oceanus, and Armageddon farmable. If you don't have the money to buy a Mythril Drone, you can just farm one from the Mines, no problem. This concept needs to be expanded even furthur to include more than just the small selection of things that are currently in the Mine Shop.
For a low level player, this really isn't a problem. But me? I'm highly ranked in both my Arena and in Tournament, so I need to go against Tauruses, Aquariuses, Alices, Great Mystics, etc in almost every battle. I still win about 60-70% of the time, because my decks are fairly solid, but that doesn't mean that I am at all at an advantage (that unintentional alliteration though) during the battle. Having these cards could enable me to go from hovering around 10th to holding a solid spot at 1st.