It does need to stay alive for you to get value out of the cards, but the good news is that there are other ways to mill yourself in UG, and turning every permanent you mill into a card effectively in your hand is just absurd
You can use cards like Land Tax and Tithe to ensure you hit your land drops, and Knight of the White Orchid will put a Plains card from your library into play
Its pretty simpleeven if wed love to play with Gaeas Cradle, we should be happy to play at a lower power level and forgo the proxy depending on the context
It's very strong at the kitchen table with its ability to destroy any artifact or enchantment on arrival to the battlefield, and it's a may effect so if you are the only one with it and you just need the body you can drop it
Anytime any creature dies, yours or your opponents, you get to drain two life from an opponent and heal yourself two, putting some heavy pressure on your opponents