Why does anyone think a seven round mag capacity will necessarily be declared unconstitutional? The ten round limit has not.
One way to look at it is that 10-round magazines are readily available. In the eyes of many, limiting handguns to 10 rounds doesn't place a huge
imposition on gun owners. In places like New York, most folks already have them by default.
Now, switching to 7 rounds will require new manufacturing and tooling. Such a law will require people to dispose of or relinquish property they already own without compensation. Serious 5th Amendment issues abound.