At high levels, DE is very unbalanced...and why is it unbalanced? Because the people with nothing to spend DE on who maxed their heroes already took advantage of the situation where people can't dump DE into walls lol.
Elixir for walls is quite a bit different, because there were LOTS of people who had maxed out their elixir upgrades for a given TH level FAR before they had maxed out gold upgrades. The proportion of those folks to the proportion of folks who max their heroes prior to maxing gold and elixir upgrades isn't nearly the same (and to SC, the change made sense...because at lower levels people were stopping their progress in order to farm gold for walls before advancing, so to incentivize advancement which is in SC's best interest, they let you spend your elixir on walls too).
How does dumping DE into walls incentivize advancement if we're talking about people who are maxed on heroes? It doesn't, they're already at the last stages of base building, and in fact it actively discourages them from spending real money to upgrade walls.

