Shiny being a guarantee that the pokemon has perfect IV does NOT decrease the odds of you finding a pokemon with perfect IVs. If anything, it INCREASES the odds. By a a horrendously miniscule amount, but still.
Unless you think we're talking about preventing pokemon other than a Shiny from being able to get 31 IVs across the board, but that's a ridiculous assumption to make.
Oh, or you could be talking about the actual stat bonuses the others were talking about. Nevermind.
I'm talking about if shinies had an additional stat boost. In other words, I was directly answering the topic question.