It is not the best solution.I think lowering the keys requirement is the best solution. Without spending gems, a F2P would spend around two weeks just to promote a 3* to a 5*. I think the whole promotion process should only take at most 4 days for F2P. This game is horribly harsh to F2P so cutting the keys requirement would benefit them the most.This subject deserves to be more active. I exposed my ideas for new players wouldn't get too many keys at the beginning, but the more you play, you more keys you get.
+20 lv = + 1 daily key
Lowering the keys price could also be good, as the topic author said.
Lowering keys requirements will benefit F2P the least, and benefit the whales the most. The whales can upgrade more heroes per $ spent. This will widen the f2p-whale gap.
The better solution is to give out more daily keys, which will benefit F2P the most and benefit the whales the least, since it will not affect the amount of heroes that can be upgraded per $ spent. This will lessen the f2p-whale gap.