It is not proven that blades of the fallen are more powerful since frostmourne is now gone, you have to be really naive to think the abilities of each of the weapons even compare, and the highlord deathknight has used the corrupted ashbringer so a DK has used a version of ashbringer.
You cant tell me you dont think summoning as many undead as you want, including powerful dragons and the ability to kill a whole raid of adventurers in one ability is less powerfull than 2 swords that hold some frost powers.