The details about T500 are still up in the air. We only got a slight insight to what Zanz was proposing. Anything could change. But the main point for T500 is for people to build very creative and good decks and have lots of people be able to play against them, and not just limited to people who are in T50. Everyone will get a chance to be in T500, possibly everyday.
So T500 will give you all the great play against well made decks. The weaker decks will get expelled over time and only the really good ones that are fun and challenging to play against will stay.
But Zanz isn't just doing this to get rid of rare farms, that's just one of the benefits to T500. And we don't even know if T50 will be replaced, and T500 will be an addition, since Zanz can do anything from now and when it comes out. But it is most likely it will just replace T50.