it's a case of bad wording.
treasure hunts and heroic level raids (farming certain creatures' medals by tapping on the magnifier icon) qualify for fulfilling the daily quest "five times the hero!“ too.
campaign is the least important part of the game for me. ok, because it's over now
but it had been the same before.
when stuck in a level i just tried some times and then turned my attention to Hero Arena, Boss Arena, Dungeon, Underworld and Heroic Raids (to get creature gems of my most important team members or most featured newcomers).
and only after i had made some progress i tried again.
don't let campaign ruin the fun for you. just forget about it for some (maybe long) time when stuck.
for overcoming certain campaign levels you just need upgraded abilities and creature stats, and maybe a new team member.
and playing campaign is not needed to fulfill 8/8 daily quests.
sooner or later you will reach the end of campaign anyway. no special reward for this btw
progressing in hero levels and making the team better and better is the core of the game for me.
and the basic battle mechanism is just fun, and again and again i have some epic battles esp in Treasure Hunt, Rainbow and Hero Arena.
a longterm engagement? certainly, and i'm glad about it. it's easy on my time and not overdemanding. no stress, just relaxing fun.
what may the future bring? new campaign area (not so important), new events and (who knows?) real-time online PVP like in Primal?
[recommended: check out Primal for a stunning and fresh approach to match games]
btw i lost interest in Boom Beach, Space Miner Wars and Dungeon Boss in the very same moment when i had outmaxed all installations and troops.
i guess that won't happen for Solgard in a long time to come.