As a returning player to HS after 2 year break, I've just tried duels couple of times and it looks quite cool and fun. However I get the impression that its a dead game mode and mostly because of the first impressions it gave me when trying it out a bit. First of all its styled as arena and not a ranked game mode in which you can just enter and play it for free. The fact alone that you have to spend gold to participate and earn rewards makes the duels a no go for me.
The fact that the option to spend gold/money to participate even exists makes the "non-committing" version of it obsolete and a waste of time. I mean why would I spend time on that part of HS when I can push for ranks on ladder or even battlegrounds ratings. And then why would someone who is new to the game or duels in general want to give up precious resources to compete in a game mode they're inexperienced in when there's the good old Arena that everyone can easily understand.
All in all what I wanted to say is the way duels is designed (not the actual game mode but the approach to the game mode) isn't appealing to me personally and I feel like I'm not the minority either. I'm not even completely sure how it would be fixed either other than removing the whole spend resources to get rewards part and adding some elo/rating system in it but even that fix would probably anger more players than anything.