Just a pondering. Purely from an aristocratic onlooker pov (I'm not but if I was ....!)
Kate was a commoner by birth. So her kids are now only half royal. If they then marry someone ordinary too then the same in subsequent generations you get to an ordinary family with one royal ancestor - William- ( except in this case they will have kept the chain of office / power of monarchy throughout ) But are they now really royal in the sense the current queen is? If they aren't is that problematic? I know that's why previous royal generations kept marriage within their own extensive royal European circles.....