Gravity exists, it's there. You don't have to pay extra energy to gte gravity to work. It wouldn't be fun if all cards drained 1 gravity quanta per turn now would it? Besides, changing it to gravity won't work, that'll just limit it more, as then the only decks with card draw will have to include gravity AND time, which is a bad idea (Precog and dial don't count as card draw, they are cantrip, and just replace their spot in a deck, and Golden Nymph isn't being counted atm because of it's rarity) empowering rainbow and completely ruining mono-time, card draw was it's best aspect.
And if all the rainbow's didn't have time mark, they would likely go to entropy, to fuel the novas. No matter how you change it, there will always be a most-effective mark for rainbows and most people will use it.