Do you include partial mash as an extract brew? If so, it's hard to acknowledge much difference between those and an all-grain beer.
However, it seems like a lot of support for equality between extract and all-grain brews. I agree in regards to that potential to a point, especially as Erockrph states in regards to a limited number of styles or their sub-categories.
Still I think that if you were to look at a wide sampling of many all-grain brews made by brewers expert in both recipe design and brewing, compared to many extract beers (excluding partial mash) made again by experts, I believe that the win column will contain a considerably higher percentage of all-grain brews.
This is attributable in large part to more control of 1) fermentability of the wort which also plays into yeast selection to reach desired finished body, mouthfeel and flavor of the beer, 2) more potential for creating a desired or creative wort profile as designed by building a mashed grist with potentially numerous malts/adjuncts/other, in comparison to a standardized extract combined with steeped grains, 3) better quality control over freshness of ingredients, 4) control of the mash environment (water treatment, grain/water ratio).
I admit some of this is based on hypothetical equal conditions, but I think you have to look at real world ability to control the variables by the brewer.
Still, I must admit there are some truly wonderful extract brews. But having gone all-grain a number of years ago, I have no desire to go back to extract beers, even though I place absolutely no stigma on them, and made and really enjoyed a fair number of them during my earlier years of home brewing. Cheers!