I don't see why they don't do it destiny style. Week 1 10 challenges, week 2 10 more challenges (that doesn't lock out week 1). That way catch up is always there, and if you're struggling with challenges on week 8, instead of being locked and rolling for 2 you can do, you could just do any of the previous.
My only guess is they don't want someone coming in not playing for 9 weeks and blitzing all of it on week 10. I suppose their current methodology is to try and keep players logging in.
Regardless, this whole thing has been handled and communicated poorly, and I can definitely foresee issues with catch up on these permanent acts.
I also think whilst these acts are good because they're easy, I also believe they reduce variation especially if replacing acts. As they were as additional acts was fine, although if DE wants players to keep logging in and not burn through nightwave too fast, then maybe the compromise instead of replacing acts would have to keep them as permanent acts that just gave less standing, such as 3000.
DE has been putting out some better decisions lately, they shouldn't ruin it now.