Uh....all that buildup about how Eraqus was the perfect foil to Xehanort inside Terra, and all it leads to is................an inspirational speech? Really?
Did you even see Blank Points? Terra was calm, collected (then again, all his anger was forced into his armor), and 100% confident that he would win the battle for his body/heart/soul/etc. You don't get that kind confidence if you believe that all your "guardian angel" is going to give you is a pep talk in the future. Also, Xehanort, even when mentioning his "other roads" (a.k.a. the 13 Darknesses) at the end, did not seem fully certain that he could win over Terra after realizing that Eraqus was assisting him. If your theory comes true in a future game, then Nomura just wasted our time with Terra's "epilogue."
Furthermore, DDD reveals to us that Organization XIII was supposed to be a group of vessels for Xehanort, but nearly all of them (except Xigbar and Saix) became too free-willed for Xehanort's heart to possess, which is why he had to create a 2nd Organization, the 13 Darknesses. The same 13 that had to have Sora's heart too damaged to resist before Xehanort even attempted to make him into a vessel. Now why do you think Xehanort was going through all this trouble to make sure his vessels did not become too independent? Perhaps, sometime before DDD, Terra ended up "showing him the door" after all.
Having Terra NOT be one of the 13 will ultimately give both him and Xehanort credibility: Terra, for making good on his promises against Xehanort, and Xehanort, for being crafty enough to get spare vessels so that Terra's victory does not really hinder him that much.
I can see it now in KH3:
Xehanort: *sarcastic clapping* Congratulations, Terra. You made good on your promise to "show me the door" after all, albeit with help from your fanatical master. However, I have also made good on MY vows as well. Your victory over my control is a shallow one, as you were not the only one to partake in my power. Did you really think that I would risk losing my chance at immortality with YOU? No, I needed an ace in the hole. 13 aces, to be precise. Perhaps my other followers will appreciate the "gift" that you spurned out of stubbornness. I have no further use for you, Terra. Enjoy your freedom, for what time you have left.