First of all, I don't think all these spin-offs like Kingdoms, Duel, Clash, etc., are harmful to the series. It's just a way to make money, nothing more. If it's well-implemented, then all the better.

This would only be beneficial if Duel of Champions were more popular than Heroes 6. But the game is unlikely to become more popular. Because, in my opinion, it is aimed at an intelligent audience. And Heroes 6 has more beautiful graphics.

No, what's simpler and less costly: creating cards or tweaking the mechanics, fixing bugs, releasing a bunch of patches, etc.? For some reason, it seems to me that cards are easier. There definitely won't be any bugs to deal with – the mechanics are relatively simple, and the art style has been accepted by the community, so there shouldn't be any problems with it.
As for the intelligent audience... Aren't Heroes games for an intelligent audience either? It's a rather strange criterion.

And in general: why does everyone think that casual games are bad?

I don't know. For me, the bad thing is that instead of creating a normal game, a continuation of the series, Ubisoft started creating all sorts of "apps."

But PR only brings results if there is a high level of interest in the object of PR from the community. If players don't crave the game, no matter how much you promote it, there will be no point.

I don't understand what this is about. We're talking about Heroes of Might and Magic, one of the most legendary series. By the way, after all the improvements, the 5th installment eventually became very successful. So, the demand was clearly there.