Why is that such a "welcome change." Why?
Why is having ones bloodline mixed away with an alien
race of people so fantastic and desirable?
Because it beats believing that marrying "an alien race of people" is evil. Or disgusting. Or that the offspring are then impure and inferior.
There are arguments against this marriage. But the mixing of bloodlines isn't one of them. If anything, it is a plus - the Royal Family could desperately use some 'fresh blood'.