Originally Posted by
hdm
Given the huge investment in game analytics SC has, they almost certainly noticed a non-trivial drop in gem purchases due to people being able to 'too easily' upgrade walls (gem purchases rise and drops are often only indirectly related to other areas - I'm not stipulating that users were using gems necessarily to upgrade walls). Walls became a good dumping ground to ensure that your base was as unappealing as possible before you went to bed/work.
SC's official reason may be one of balancing the economy, but this December's update/correction was surely precipitated in large part by a drop in gem IAPs this Q4. Also regarding balancing, no such balancing would be required if both non-premium currencies in the game were equally useful, and equally useful along the same timeline (i.e. yes, lab upgrades require a large amounts of elixir, but over a very long period, allowing you to always be running a surplus typically). Moreover, after all buildings/troops/spells are upgraded, you are done with elixir (except for armies of course) - BUT you have 10s of millions more gold that has to be farmed for walls, all the while with full elixir coffers.
Regarding "why don't you just create really expensive armies now that you have all that elixir": this would be ideal, except for the massive cooking and barracks time...not practical (see timeliness-cost comment above).