I think the author of that article is a nut. I have seen US stamps, as well as those of other nations, with Mickey Mouse and Donald Duck on them and no one thought that was the elevation of cartoon characters to 'Royal' status.
The USPS is in bad shape (like other PS in other countries) and I think it is a good thing for it to try to get itself out of the problem by legitimate means...rather than asking more tax-payer money.
It's just another article by a Doom-prophet and as such should be filed in the circular file. There are many more pressing problems in the world today than worrying if Mick Jagger is going to be on a US stamp or not!