In my opinion devs tried to put in the game players idea about fixed rifts and they failed at it.
Biggest problem of challenge rifts is that they are totally disconnected from your char. In the game that is all about character progression and item grind we got a mode where we don't use our beloved char ... what ?
This could be a really nice addition if they would snapshot some rift once a week, then make weekly leaderboards and allow people to compete for those weekly rifts but with their own chars. That would remove grift fishing and would introduce creative ways of playing our current toons to squezze maximum of those static rifts.