It wouldn't just be an eb though, some would last for days. It'd be really tough for some clans to complete and the higher tier ebs would be even harder of course. Mith is a nice reward.
It's still an eb...just a really long one. And mith is for war, not ebs. People work hard to get mith in wars and to UG war equip. Don't bring it into ebs. Keep it to extra bonuses on what ebs already give. There's less incentive to war as it is, don't affect wars even more.
I guess you're right, would be ok for a limited thing but mith is the only reason I war. Extra plunder, aqua and inferno would be great as well as clan achievements and bonuses.