We certainly can define "moral progress" that way, if we wish. It's just a phrase, we can define it any way we like.

But we should take care, after so doing, not to assume that the properties we would naively associate with moral progress apply to the referent of "moral progress."

In particular, a lot of people who talk about moral progress seem to believe it has something to do with people getting better over time by the speaker's standards. If any path involving one-way ratchets is by definition moral progress, then there exist paths of moral progress that involve people getting worse over time by most speakers' standards, so they ought to give up that belief if they're going to use that definition.

It may be more productive to use a term less subject to misunderstanding.