Against this proposal, it limits to many cards that would not be used often but are used as clutch monsters in certain situations such as exploding dwarf and exploding rat.
For some, it may not be worth the level-up of an exploding rat or dwarf for the extra abilities if they only use it in an opportunity+ equalizer game
That said having a level one sitting around for those particular rulesets would be handy and shouldn't be penalized as part of regular gameplay.
It may also cause card concentration as new cards would need to be maxed to a certain point to earn rewards and disincentize rentals.
For Example, A GF Legendary Alpha under this proposal would be penalized unless you merged it with another card. Such as a GF Gold Dragon with a second GF Dragon.
I would not support penalizing Alpha Pack Holders of rare cards to upgrade those units.