Level 7 walls need to be able to be upgraded with elixir.
Many townhall 8's and 9's have already upgraded all that they can with elixir. This leaves them with a pretty full elixir storage for the rest of the time they are trying to upgrade other buildings and walls. Elixir becomes pointless as it can't be used for anything except troop training. If you could use it to upgrade level 7 walls to level 8 walls, this elixir issue would be solved, making the monotonous task of upgrading walls a lot less painful.
*This is an addition after reading some of your comments on the subject. If walls were upgradable with elixir from level 7 to level 8, there would need to be a minimum required amount of elixir to have in your storage to be able to spend it on walls. This would prevent the main problem most people see with this idea which is the lack of elixir in the economy of the game.
Please reply with thoughts on this idea.