Knock down requires you to hit with a regular attack, and then requires you to make the touch attack to trip and make the check. Epic Trip allows you to make all your attacks (on a standing opponent) as touch attacks that trip and deal normal damage. So there's more benefit in epic tripping. Also, I don't see the "auto trip anybody standing up" thing being overpowered; you still have to hit with the (touch) attack, and you are more than likely able to near automatically make tripping checks if you specced for it.
To clarify a bit:
Knock Down = Make a standard attack for damage, then a touch attack, then a trip check.
Epic Tripping = Make a Touch attack for damage, then make your trip check (not needed if you hit an opponent attempting to stand.)
So, you're of the opinion that it would simply make Knock-Back pointless. OK.
Personally, I reckon the feat's far too good even as an epic feat. Making melee attacks touch attacks by itself would be a borderline-overpowered epic feat; all of the rest is just getting silly.