Your whole theory is based off of dropping 360 sales... but year over year the 360 has increased while the PS3 has decreased.
A main reason that I don't think 360 sales will get hurt too badly despite lower tech is because multiplats are usually the best selling games each year and 9/10 multiplats run equal on both platforms or run better on the 360. They also sell more on 360. I don't think developers are going to go out of their way to maximize the PS3 version when it's the weaker performer of the two. Not because they hate Sony, but because it doesn't make any business sense to do so and at the end of the day, these are businesses.
Also, multiplats have great graphics. It's not like PS3 exclusives make multiplats look like sloppy productions or last gen titles. Even when games are specifically made for the PS3, they don't look that much better than games that are made for both. If the PS3 is seen as pretty much being the same as a 360, I don't think 360 will suffer to badly especially when the 360 sells for half the price. Sony's price drop is a rumor so far and we don't even know what it will be. If they drop it down to $349, I don't think that's gonna have much of an impact. MS will probably just drop $50 of their sku or do something like the elite bundle where they give you two top rated must have games (Halo 3 and Fable 2) while Sony either offers no bundle or they throw in some mediocre game that did sell well (ie a game that people weren't interested in buying to begin with).
Another thing, as time goes on tech gets cheaper. Blu-ray is becoming less and less of a selling point. I can goto Best Buy and find players for $150-$200. When the price drops down to where you can find normal ones for $99, I think Blu-ray totally stops being an issue. Especially if it's cheaper to buy a 360 and standalone blu-ray player than it is to buy a PS3.
