The beauty of monuments is that you can get an unexpected winner who used their brain rather than strength, or the favourites can lose with bad tactics. Cycling is a sport, no a fitness test.
However at the same time these races are 250km+ and hard, so everybody good enough to compete for the win in the final has to be exceptionally strong before tactics come into play.
I don't think a colourful list of previous winners loses a race prestige. Coilek beat cancellara and sagan , Gerrans did the same and mugged valverde/Dan martin in Liege. Hayman out sprinted Boonen. These guys didn't decrease the prestige of races, they improved massively their own legacy.