This isn't correct. Because the change in your rift rating is determined by the relative rating of you and your opponent, if you win/lose at 50/50 with people that on average have a higher rift rating than you, you'll still rise, because you win more points in the 50% you win than you lose the 50% you lose.
I thought about this and I don’t agree. The case described assumes that the average rift rating of the opponents would rise as you improve your match making rating. But this discussion is specifically about decoupling the two. If you can face players with higher rift rating, then there’s equal chance to face someone with a lower rift rating.