AMD may beg to differ, and I'm sure they've done a cost-anaylsis that says you aren't "worth it" to support, and maybe they are even correct from an economic point of view.
AMD is a tiny company and they have to pick their battles. There is no question that supporting W10 well is much more important than supporting W7 and W10 less well.
Remember guys, it doesn't matter if more people are on W7 presently. What matters is what *new* processor buyers will use. That is overwhelmingly going to be W10.