Fair points you make! Immutable announced at some point that they will ban people intentionally losing, but as you said, you arent losing intentionally, just not ranking up.
Its really hard to make the reward system fair. I think if you consider both wins and rank, there will always be a certain sweet spot. Maybe your suggestion would help a bit, but it wouldnt solve the problem. In Hearthstone, only your rank mattered and I disliked the system even more.
Maybe you could give out trinkets for every win in mythic at the end of the month or something? Like cardbacks, statues, etc. This way every player could decide on his own whether he wants to farm cards or go for that sweet tradeable but useless thingy...just from the top of my head 😅