I think milestone videos should be done not too frequently, where everything becomes a milestone, but intermittently. You can always thank your subscribers and viewers in videos but making a milestone video is like having a celebration. If I were to make a milestone video, I would think, in the overall scheme of things, if that particular event is going to be something I look back on to say, "I did that? Wow!!"
A celebration every week (50 views...100 views, etc.) wouldn't feel like a special event to me anymore. That's just going to hang out--like that one house on your street that has a "party" every weekend.