As a paragon level 100 wizard with top tier gear, here is my perspective:
Archon is spectacular for clearing low level MP. With MP as high as 6, it's relatively easy to keep archon up almost indefinitely. I would not contest that archon is equal or better than WotB on low MP levels. The lower movement speed is annoying, but that would be better dealt with through a change in the movement speed cap allowed from gear.
The problem with archon vs WotB is that archon scales poorly on high MP levels. With WotB, the barbarian is unfazed by affixes such as frozen (a killer on archon where you are instantly dead if you lose the life steal your damage provides) and is also capable of keeping WotB active in high MP levels (Fury generation is not harmed by higher monster life but archon requires constant kills to maintain - which is extremely useless in high MP levels.)
I would hate to see WotB nerfed - I think it's exactly where it should be. I would much rather see archon see a boost that causes it to be more useful on high MP levels (Some method with which to increase the duration that is possible to sustain on high MP levels) as well as better options with which to combat CC (Maybe add 75% reduction to archon).
Obviously given my character I'm quite biased on the matter, but I do not think I am incorrect in asserting that barbarians have a distinct advantage when it comes to high MP levels.
What would you think of maybe increasing movement speed cap by 25% on Caster/Range classes? Seeing as Monk + Barb have -30% damage reduc, what if range classes could boost 25% more move speed?
It'd add alot of options to builds because of rings/legendaries with move speed on them