Joe Montana = System QB
The System? WCO.
His replacement? Went on to win Super Bowls in the same system.
When Joe left the system, he sucked at KC.
(This is just another possible frame for the discussion).
I hear what you're saying, but I don't think it's fair to Montana to say he "sucked" in KC, and that it proves he was just a system QB. His career arc actually followed a very normal curve. Look at his QB ratings (as one measurement) for his career, starting in 1980 (and in 1991 he missed the entire season, and in 1992 he played 1 game, so we'll leave those out as well):
1980 (24) - 87.8
1981 (25) - 88.4
1982 (26) - 88.0
1983 (27) - 94.6
1984 (28) - 102.9
1985 (29) - 91.3
1986 (30) - 80.7
1987 (31) - 102.1
1988 (32) - 87.9
1989 (33) - 112.4
1990 (34) - 89.0
1993 (37) - 87.4, with KC
1994 (38) - 83.6, with KC
1986 was, if you view this as a curve, an oddity (he did only play 8 games that year), but in general you have this pattern:
Early years (ages 24-26): ratings in the high 80's
Peak years (ages 27-33): ratings in the 90's and 100's
End years (ages 34-38): ratings declining into the 80's again
Pretty standard curve, really. Nobody's career curve is perfectly smooth, but this fits what you'd expect. He started off pretty good, became phenomenal, and then, as he hit his late 30's, declined into an average QB.