I think that
the C64 line took a fast nosedive in 1987 - never dying,
 but definitely not a strong seller. 
 I think the problem is your sample environment. The C64 still sold 80,000
 units in 1989, building on the already huge extant userbase. (Reference:
 
http://www.indwes.edu/Faculty/bcupp/Things/Commodore/Development.html) 
 
Whoops, that was supposed to be "Christmas" 1989.
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