This has been considered many many times in the past. Unfortunately there is no way to add it such that it is not broken. It either is worthless, removes rarity or gives infinite . Any of these three results would damage the game.
A worthless feature doesn't hurt the game, getting rid of rarity would make the game better, and infinite electrum almost exists already since the game gives it away like candy. 2 out of 3 aren't harmful, 1 of the 3 is an improvement, the other doesn't make much difference.
Getting rid of rarity would shrink the target audience thus damaging the game.
Same holds for infinite
(people that want infinite
use the trainer instead)
Worthless features take coding time from valuable features.
All 3 are harmful.
Note: Rare cards should not be required to play the game.
Sidenote:
The rares are not rare for you Uppercut. They are rare for the collectors and grinders.
The trainer is not there for you. It is there for the grinding adverse players like me.
By appealing to the broad range of players the game creates more enjoyment than by appealing to only Uppercut clones.