One datapoint on price... The spider version of the 720 is, according to the rumors in the 720s section, a few years away. The Sports Series spider might be unveiled later this year, but it may not be available until next year. So, if you want a convertible McLaren this year, you're looking at a 12c or 650 (or LT if you're lucky!). If you want a new one next year, you'll be looking at the sports series.
Given that the vast majority of 2013 and later P11 cars are convertibles, maybe the hit from the 720s isn't so bad.
It's also a fair point that the P11 cars have already depreciated quite a bit. It's entirely possible they continue to lose value, they are cars after all and that's what they tend to do, but I doubt the 720s will rock the P11 market much more than the standard damage done by time and miles.
“I had nothing to offer anybody except my own confusion.” ― Jack Kerouac