Sooooo, about SDI, do you really think it was a real threat? Observing US's struggling with ABM now (which is a mush humbler project than SDI was), watching its doubtful effectiveness, I doubt US was capable of SDI in the early 1980s.
Some say that SDI was a bluff in order to hurl USSR into more expenses it could afford, well, if it was it was a very successful one, I'd say the Soviet leaders believed it. Still, I don't see how KGB missed this fact and generally I cannot imagine a bluff of that magnitude and I find it hard to believe.

I think USSR was destroyed from within, not from outside. By its own people, not by some clever US president.