I understand what you're saying; I just happen to disagree completely.
First of all, it's unclear who Obsidian designed Quest mode for, and I don't think that you know given that you don't have a Dev tag slapped on your profile. One would think it would be for the same people who enjoyed Story mode and want more. I sense that there is some sort of implicit accusation I'm not hardcore enough and always hate grinding - let me dispel this notion by noting that I've got hundreds of hours on Disgaea games doing nothing but grinding. Slow grinding is not a sin of game design unto itself; slow grinding with extremely infrequent rewards, on the other hand, is a paradigm that is rightfully hated by most gamers (ref: pre-loot-2.0 Diablo 3).
Second, I feel like people are not doing the math here. If it takes three games/wins to increase in level, and each of those games takes 20 minutes, then by the time we hit the eventual presumptive level cap of 70, that is 70 hours of gameplay in Quest mode alone, and that's assuming something of a best case. If it were to take five sessions (3-4 wins plus 1-2 losses), you'd be at something closer to 115 hours. And, doing some averaging here, if ten levels takes ten hours, I am getting a meaningful advancement for my character roughly every 3-5 hours (7/10 of the rewards atm usually being trash cards). These would be punishing numbers on a table, and they are just as punishing on my phone.
Finally, the idea that Obsidian must turn Quest mode into a slow grind to stretch out the content is basically saying the designers suck at their jobs, and had to artificially generate more gameplay hours to make up for a lack of content. I don't think that's the case here - I think they rushed Quest mode, and it still needs significant tuning. If I want to stretch out Quest mode, I'll play it on Heroic or Legendary for more challenge. Don't punish me for trying to access more content at a rate that a normal gamer would find gratifying. I'd much rather they make Quest mode three times as fun and take half as long to reach cap.