I understood most of this intuitively, but I never thought about the "you might win it later" part for upped cards. I upgrade way too aggressively.
I just now reached a new personal record of 14,000 electrum, but only because WAR is coming up and I want to have upgrade versatility. Were it not for that, I'd probably have spent it all on upgrades I don't need for decks that I want to "try out".
Thanks kevkev.