When something is taboo or even just unusual, it has impact if you do it. Just as boobs on screen used to have a certain effect but now they're in pretty much any movie intended for adults, showing penis will become mundane and lame if everyone starts doing it. That would be too bad. There should always be a final frontier remaining. So I hope it doesn't happen, but I can see how it might.
That said, I don't know exactly where the trend will settle. True, there are a few major actors who are already giving everyone a good, clear look, such as Ewan McGregor and Peter Sarsgaard. And the penis is being featured more prominently, like the two naked guys in Hall Pass. But I don't think it's becoming an epidemic for major stars to really show what they have. Look at the examples of A-list male full frontal that we have now. Richard Gere, Mel Gibson, Gerard Butler, Aidan Quinn, Gary Oldman. They show their cocks, but it's just a glimpse in passing, or obscured by distance, movement, or angle. Half the time you see a movie everyone is talking about because some big actor's penis makes an appearance, and it's like, "That's it? Everyone's shitting their pants because you see something or other swinging for half a second in low lighting?" It's not like everyone is getting naked and standing there in all their glory for you to check out their package. It's titilating but only that.
And I don't think size will be a big deal even if it does become the norm. Contrary to how it may appear at LPSG, which is intended as a meeting place for above-average men (and even here, we all know most of the claims of 8+ inches are a little exaggerated), most guys aren't that hung. We see naked celebrity pictures here all the time, and they're usually not that impressive. They're just people, and most of them are growers who reach five or six inches at full mast.