I actually suspect that pistol calibre carbines is the exact reason why they are limiting all magazines to 10 rounds.
Effectively people were using the 10 round pistol mags as a loophole to get themselves a 10 round long gun - despite the long gun "5 round magazine" limit.
Whenever people push at loopholes there is a correction. This time it looks like they may have take it too far.
You should clarify your statement or do some homework. There is no 5 round limit in mags for long guns, per se. Can buy larger capacity than 5 round mags in quite a few calibers. There is a bunch of fine print in this and a lot more grey area that people misread.
I was using 13 round mags in my ruger PC- 9mm; legally as well. Like using 25 round mags in my 10-22's. There are legal ways.