No, there is no way to downgrade a ps3 unless your flashing the nor flashdrive, and the e3 is the only known way to do that.
There is some services at ebay and maybe craigslist. They charge 49$ to get your 4.20 downgraded and jailbroken to a cfw of your choice.
But, the e3 team has the 4.21 private keys, so they might release it in the nearby future.
The 3.60 keys wthat was released was the public keys, not the private ones that leads to a cfw, sony also patched "Booting" So there is not going to be a 4.21 dongle, atleast. Sony couldn't care less if you gain the 4.21 public keys too, that only allows us to play 4.21+ games.
But if the 4.21 private keys gets cracked again, if so, someone will make a downgrade and the ps3 scene will hot up again