Mostly everything you said is what I mean except the not-Buffy thing. She is Buffy. That's the problem.
Buffy dies in Season 1. Line continues through Kendra and Buffy is dead to the line. That's why her death in Season 5 didn't spark a new Slayer.
Buffy is brought back. Since being a potential is something put into you at Birth, Buffy being brought back meant she is put back in the Slayer potential line.
Kill all the SIT's plus Faith and Buffy is the only 'potential' left. But since she already has been a Slayer, there is a once only rule and the power stops.
Now while there could be more SIT's, the power can never get to them since the its through Buffy, but Buffy can never actually recieve the Slayerness.
Power stops exsisting since Buffy will never access it and even if she dies a Third time, since she never got the power, it can't be passed on to a new SIT when they are old enough to recieve it.