If you believe this spreasheet, (i think it's Forty's), then even at a 5% nerf, mythic t7 obsidian is still weaker than 3 grades of erenor, and barely on par with an epic erenor.
adding crit damage to erenor in addition will make mythic obsidians useless, and everyone with one will be stuck with a bound-forever item that cost a ton of gold and is no longer BiS.
unless they make it so that you can feed high tier/grade obsidians to erenor for a very large amount of exp, or give us the option to unbind obsidian gear, i think nothing should change from the devs current plan.