A couple thoughts:
1. From what I understand, you can still use 10 round magazines if you have them, you just can't load them past 7 rounds. :banghead:
2. By "New York Law Enforcement Officials" I assume they mean police chiefs, who in many places, especially New York, are more politician than Police Officer. You have to remember that the International Chiefs of Police and the FOP often disagree on subjects, especially guns. This is probably not a rank and file opinion.
3. Seven was probably chosen for two distinct reasons:
A. They've already done 10, it didn't work, so clearly the magic number is lower.
B. Outlawing all semi-auto pistols with a mag limit might put them in danger of having their law overturned by citing Heller's use of Miller to establish protection for weapons "commonly in use in the home" for self protection. By going with seven they can say "See you can still have 1911s and S&W Shields and many other similiar single stack semi-autos."
4. I imagine that large caliber handguns may suddenly become very popular in NY. If you can't carry more than 7+1, then some of the arguments in favor of 9mm over .45 sort of go away. Not all of them mind you, but some, which could cause a resurgence in the popularity of large caliber handguns.