saamThe problem is this gives an even bigger advantage to teams attacking 4th/last points. Imagine you are holding gullywash last with even ubers. You have to sit back while you build your uber because going too aggressive in lobby is an easy way to get wiped and lose the round. Unbeknownst to you the other team spawns a heavy and puts him on top of second point, and at this point you've already waited around for a bit after getting an uber expecting a sac. At this point it doesn't matter that the enemy team also doesn't have uber, they have no chance of a successful push out of last when they have a weak or no uber to get thru chokes and force an even fight. I think defensive offclasses are a good way of making sure that while winning mid gives you a good advantage in a round, it's not a free win, but this change would mean you can just wait for both teams to lose uber and then do whatever dumb shit you want (slow pushes on process far right with a sniper where they can't uber trade into you, bombing soldiers on to point on gullywash and walking a heavy in up to, etc)
As it is, if you want to push last you either need a sizable uber ad or you need to keep sacing players until you create one, or give up and go for the sniper. I don't see why uber degredation makes bringing a heavy to push into last any more viable than before, but even if it does, I feel that giving attacking teams a reason to push (as opposed to standing around waiting for picks) is a positive change.
More importantly, I don't think that teams actually losing uber would be a major factor with this change, as long as the uber drain is slow enough. What this really encourages is players getting caught out. In your example of holding Gully last, yes, holding your whole team in lobby is an easy way to get wiped--but you don't really have to do that. In order to maintain your uber, both teams would just need one dude standing close enough to consistently take spam. This isn't enough to guarantee a team gets wiped trying to maintain uber, but it does give the other team a better reason to play aggressively, trying to catch that risk player.
Sure, one team could decide to just sit around and not spam, but that would let the other team get really close and execute effective sac plays.