First, no matter how good your defenses are, if you have full storage a little bit of zap from lightning spells let you lose great amount of resources.
Second, there's really no need for spells to damage non-defensive structures? I can consider if there would be a spell which would amplify damage to structures because it would still require some troops before it would be destroyed but no one actually spends lightning spells on empty storage so they could save time.
Possible solution(s):
a. Grant spell immunity to non-defensive structures which would mean they won't get damaged by spells.
b. Allow only troops to acquire resources from structures which means even if they damage structures with spells, they gain nothing from it.
c. Emphasize alternative ways of gaining DE. for th level 7, a level 3 DE drill means 540 DE over 12 hours while 3 level 4 lightning spells against a level 2 DE storage with over 1000 acquirable DE could give about 400+ DE at the cost of 60000 elixir and doing that for over 12 hours means several thousands of DE given that the player have more than enough elixir. Clan wars also give reasonable amount of DE which is great. Balance them to a certain degree.
I don't know. There isn't really a scarcity in DE, maybe it's about the scaling of the need for DE and the capacity to hold it like in TH7 you can have a minimum capacity of 10k DE and at most 20k and first you need 10k for a BK which is equal to the capacity of a level 1 DE storage then after some time to upgrade it to level 5 you need 20k DE which is the maximum capacity you can only hold at th7. My point is, what makes it so hard to gain is because you'll get constantly attacked when you have reasonable amount in your storage. Plus players zapping your storage do not even give you shield to recover the resources you have lost. It makes anyone furious that they cannot avoid losing resources to this kind of players since there is no way to defend your storage from spells.
I hope you'd find a solution for this problem that has existed for so long.

