5000 ED is better than 20% ED epic affix but mythic effective works differently. It makes all your attacks deal 25% more dmg to the enemies that usually resist their own element. Effective can work with ignore resist or weaken as well. Instead of 20% ED as epic affix, it's better to use legend 100% ED as well as 30% from 6× of all elements nature.
In general, 5000ED isn't very useful in the DPS formula at least for Campaign (PvE) but what is useful is 100 WD%, many 100% ED and 5000 WD usually. For PvP, flat ED is the most powerful DPS boost for the formula. Still a pet with ED of some form is always good.