You say "infinite" and I know your intention with using that term in this argument is to make is sound massive, but the more accurate context is that while elixir is forever needed from an attacking/offensive/playable standpoint, it is needed in much, much smaller amounts than gold. I'm talking fractions when compared to the amounts that can be farmed using those troops built with it each raid.
Also the xbow refilling cost does not apply as TH8s cannot get xbows, and elixir loses it's "unneeded" value tag the moment you become a TH9, whereas it still keeps that tag from TH8 and below.
So simply allowing lvl 8 walls to be done with elixir but at double the cost would be an acceptable move. Doesn't hurt anybody, and TH8s get the chance to work on their skull walls (even though they could be farming walls more efficiently as a TH9, but that's their choice). There is no downside.
I would also be in favor of this. Even making it 10mil would be ok with me.

