In order to get a new root method, "they" need to find another exploit. The exploit that was used before .21 was patched by Asus.
If you want root NOW, you need to unlock bootloader and load a rooted custom ROM.
If you get your hands on a brand new TF201, then maybe it isn't updated to .21 yet, and you can ignore the update message, root and install ota rootkeeper. Then it should be ok to update. I have read that people have kept their root this way.