Does a Special Cuvee Degrade a Producer's Basic Wine?

In the thread regarding old CVNEs Ray raised a question of whether the quality of Imperial has deteriorated after the introduction of Real de Asua. More and more, I hear this type of proposition stated as a fact rather than a question. I've heard people say, for example, that they will not buy Prado Enea after the introduction of Torre Muga. They are not saying that the like Torre Muga better, only that they are convinced that better grapes that would ordinarily go in to Prado Enea are being diverted to Torre Muga. In most cases, I find this very hard to believe. With respect to CVNE and Muga, for example, I would think that size factors would make any difference infinitesimal. I also presume that in many cases the fruit quality between a vin de garde and the basic reserva is essentially the same, and only the vinification is different. Finally, it seems that basic economics would suggest that a producer would not rob quality from the basic wine -- while we might gasp at some of the prices of the special cuvees, far more money is made (or lost if it doesn't sell) from the basic wine due to the extra volume produced.

Others think I'm crazy and say that they simply take the producers at their word: the best vineyards are devoted to the new prestige wine and simple logic dictates that, if this better fruit formerly was added to the basic wine, that basic wine will now suffer the loss. Perhaps in some rare cases I might see a difference, for example in the Descendientes de José Palacios "Corullón" prior to the single-vineyard effort. I am inclined to think it is the exception rather than the rule. Am I wrong?