Even though we know the formula very well from the first two "Terminator" movies it still works: A T-1000 (good) and a more powerful T-X (bad)
Kyle Reese from the first movie was a human sent back to protect Sarah Connor while Arnie's Terminator was sent back to kill her. Also, Schwarzenegger is a Cyberdyne Systems Model 101 (T-101). The T-1000 was Robert Patrick's Terminator in Judgement Day.
But I agree with your overall point: the formula of a good Terminator sent back in time to protect humans while a bad Terminator was trying to kill/stop them worked even if Rise of the Machines couldn't live up to Judgement Day's reputation.
Shout by DRNKMNKYBlockedParentSpoilers2015-07-15T15:37:33Z
Even though we know the formula very well from the first two "Terminator" movies it still works: A T-1000 (good) and a more powerful T-X (bad) are sent back through time to terminate Kathrine Brewster, the wife-to-be of John Conner.
A rather unexpected ending: Judgment Day can't be stopped besides all the efforts from John Connor and the T-1000 and the nuclear first strike by Skynet happens, leading to the establishment of the resistance lead by John.