Obviously there are some implementation issues with this idea that others have pointed out, but developed properly I don't see why uber degradation can't be used as a way to encourage fights. The issue this idea tries to address is that no team wants to risk using an uber while risking being unable to pop the other team's uber, nor do they want to lose players trying to force and give the opposing team a chance to push. So an easy solution is to make uber something you have to pop.
What if uber only built while healing hurt players, and slowly drained while not healing or giving overheals? Would be even more effective at encouraging fights if self-damage didn't count towards uber.
A couple of details to consider; trying to avoid minor issues that people are gonna want to harp on.
1. While not healing, ubercharge drains at a rate of 1%/sec. I think that should be the max rate considered, slower might be better.
2. Uber does not charge while giving overheals.
3. After a certain time period of giving overheals, uber will start to drain. I'm thinking at least 10 seconds.
4. Self damage does not count as uber heals. Keeps teams from settling into the exact same stalemates as always except with one player who's just shooting himself in the face constantly
So now teams can still slow down here and there, which I think is important for both players for thinking things through and commentators so they can maybe say some interesting things, but the no-risk eternal turtle hold dies since at least one player needs to get close enough to consistently take damage from enemies and risk getting caught out.