I must admit when I started playing the game a month ago I thought the mastery system did work to make cards slightly better.
The way the +'s on the mana symbols are shown it looked like the card gained more mana from those particular colour matches, effectively becoming cheaper to cast when mastered.
Mastery was designed to encourage players to play a wider variety of cards. It’s failing at that slightly because the rewards for it aren’t actually positive (though at least it’s spreading out the player base as a result), but it’s still clearly the intention.
Your system would directly discourage people from trying new cards, because the cards are weak until they’re mastered. So I don’t think it’s likely to happen because it’s working against what the mastery sysem was designed to achieve.