LGW, I am not sure how you come to your conclusions. The same can then be said about, Oh lets see.....727's, 737's, 747's, A319-321, A340's oh, and the Gulfstream I, II, III, IV,450,V,550 and just about every other successful line of aeroplane that has ever been built. The 900 is an improved version of the original HS125, It is the natural progression in technology and development, it is an improvement on the previous models. The truth of the matter is, that only unsuccessful airframes don't have variants. Unfortunately no matter how hard you try, you cannot call the HS 125 a failure!!!
I agree that the 4000 caused Raytheon some serious problems, as did the 1000 in earlier years, They are completely different certificates. The 750, 800, 850 and 900 variants where not part of the problem, they were the stabilizing solution.