T-X, the 'anti-Terminator' Terminator, was made once the T2 storyline occured which altered the timeline, and made SkyNet aware that at some point, the humans would gain control of Terminators, so SkyNet, in its infinite wisdom created the 'anti-terminator terminator' to go clean up the timeline... even if you don't consider the timeline alterations, the machines would 'naturally' work on upgrades as long as those pesky human resistance fighters kept nipping at the heels of all the 'obsolete models' like t-800s... as humans learned the weaknesses of each successive series of Terminator H/Ks, SkyNet monitored the battles, and developed new and better terminators to make it tougher on the humans.
In dicking around with my T-4 and 5 ideas, I had developed a story where the human resistance at the Crystal Peak complex rebuild the T-850 and T-X. Well, they rebuild 2 cyborgs using the spare parts, and whatever other parts they acquire during the battles with the machines... one of the 'human-constructed' cyborgs turns evil (the one with the T-X CPU) and the other (the one with the T-850 CPU) stays good, so at the end of the T4 story, they face off again, for a juicy prize, and T4 ends in an unexpected way; the good guys lose... which sets the stage for T5, the invention of the TDE, and the closure of the loop, which ends the series... or does it??