I still don’t accept the premise that Duels was a failure.
If it was a success, they wouldn't have shut it down. And by success here I mean making a lot of money. No-one would discontinue a digital game making a huge profit to try and make the same kind of digital game a 2nd time.
Magic Duels had a model where most of the dedicated fans never had to spend any money on it ever again, while at the same time the barrier to entry after a few expansions was over $100 or months worth of grinding for an inferior experience to Paper or MTGO.
So the game had a model that didn't even give long-time dedicated fans the
option to spend money (literally people made threads complaining about having too much gold and having nothing to spend it on), while at the same time it hemorrhaged any new players due to a massive barrier to entry. WTIH NO DUST SYSTEM (sound familiar?). That is the complete reverse of what you want in a F2P game. A successful F2P has a low barrier to entry, gets you hooked, and then encourages long-time dedicated fans to keep spending money over a long period of time.
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I'm sorry, but you are just in plain flat-out denial at this point.
Edit:
Just from a design stand-point, the game as a F2P game is poorly designed.
1: You could pay for the first expansion on release and then play a reasonable amount daily to never have to spend money on the game again. It gave you stuff for free fast enough that even for free after Battle For Zendikar people who played from the beginning didn't really ever have to even consider spending money on the game.
2: If you joined the game too late you had a massive barrier to entry because of the lack of a crafting system. There simply was no way to quickly get all the cards you'd need to make one competitive deck without grinding for over a month or spending ~$100. That's too high for a F2P game. The Vault is an extremely greedy attempt at rectifying this in their 2nd iteration.
There is no way the game was going to make a long term profit with this design, which is why it got shut-down and they are now trying a triple greedy version of the Blizzard model of things.