Well, you just touched on another reason why I don't like the "what you start is what you get" way of doing things. The amount of essence channelers are given would totally determine how much channelers will be able to do throughout the course of a game, even if that game lasts for a hundred thousands turns. I don't like that. I want all my games, long or short or ridiculously long, to be dynamic, and having some pitiable predetermined amount of essence seems counter to that.
I don't see how having methods to gain essence trivializes it if done right. You yourself said you want channelers to start with their full essence, but have to research or put effort into using all of it, and pointed out that the difference between our two preferences is subtle. Subtle being the key word. In both our methods, the channeler can slowly increase his usable essence. In yours, that ability is capped out at some predetermined, arbitrary limit. In mine, it continues on and allows for natural progression of your channeler and kingdom for however long the game goes on. Game-limiting vs. open-ended.
It doesn't give your channeler unlimited essence (unless you intend to play a never-ending game), and it still forces you to use your essence very carefully because whatever essence you have, besides what you started out with, was hard-earned, and any essence that you might have in the future will be equally hard-earned.
I said reversible, but I never said easily reversible. The essence you invest would have to be locked in there for a number of turns before you use it again, and it should require a significant amount of mana, time, and maybe even money or production points to pull it out. Then there's the fact that you may have to completely destroy a magical item to get back your essence, and that a hero who has become Mr. Universe using your power is not going to want to give it back w/out a fight.....
It's still an undo button, even if it has some small print attached to it. I agree with whoever said above that essence should be totally separated from things like gold or production and maybe even mana. Investing gold to get back essence is silly and really does trivialize essence (it turns it into something that can just be bought). Having a good economy should not make it any easier to get essence, as the two are fundamentally unrelated. But yeah... I just really don't like the idea of being able to say, "whoops, that's not working as well as I thought," or, "hmm, that would be even better! Let me just wait a little bit and then throw money at it and get my essence back."