The problem with the Eureka vs farming old content for the relic thing is that by removing the necessity of doing old content for the relic you've pretty much killed most of the incentive to do it for most people.

If the relic comes only from Eureka, then what do you even do in the game once you have all classes at 70 aside from the daily expert roulette and Eureka? There's no incentive to do anything else and that makes end game really boring, especially if you happen to dislike the type of gameplay Eureka provides.