Honestly the number of legend players that will be at ranks 19-20 for the whole month is probably very low, sure maybe some are farming for golden hero so maybe a few.
This is however pretty exploitable, if this became a feature, and was endorsed, every farmer would equip their legend cardbacks and get a free win.
As for the golden hero this especially I wouldn't recommend as most golden portraits are either bots or people with large enough collections and experience that probably don't enjoy the "challenge" of farming for gold.
On legend cardback maybe. On golden hero a solid no.
