I’m personally against this.
I think it would vastly benefit spenders - there are people who have 10+ Clarissas, for example. They can retrain these duplicates (only the Clarissas, not even counting all the other duplicates) until they get a hero they don’t have - and that would be quite easy. 7 days and they will get a HotM (who they do have or don’t have). Even though the training takes 7 days, at some point they will have all (or at least most of) the HotMs they want.
It’s even easier with challenge event heroes (who are 15), S2 heroes (who are 9) and seasonal event heroes (who are 6) - they can use their duplicates to get all of the heroes they want.
Also, the Devs said that HA is supposed to help FTP - I don’t think that many FTP have non-S1 5* duplicates. So I could trade in my maxed Margaret (losing all tonics) hoping for Alberich, but I could get Zeline, for example. Then I’ll have to choose whether I want to trade in Zeline, hoping to get Alberich at some point, for example. Or that I keep Zeline but forget about getting Alberich.
I don’t see SG doing that since HA10 would be a much better alternative than summons and people won’t buy gems (unless they are targeting a brand new hero).