I am going to attempt to answer your question honestly, without any of the normal Horde responses. Clearly something is wrong with AV. Neither faction should have such a dominant win percentage. Is the position of the Horde cave actually a problem? I don’t know, but I can admit it might be. Here’s where I think it gets tricky to try to simplify and say moving the cave would fix the issue. It is clear, to me at least, that the map does have some imbalance that favors the Horde in the current meta. Maybe it’s the cave. Maybe it’s the layout of IBGY or SHGY, but something is wrong, but only for the current meta.
We know it’s only for the current meta because this exact same map saw an Alliance win rate of 75-80% between patch 1.12 & 2.2 in Vanilla/TBC when the meta on both sides was rush. We saw the same thing the first week or so of AV in Classic, and Alliance had a huge win rate. Then Horde, responding to preforms, shifted the meta towards first a hard defend and then further to scorched earth meta we have now. We saw the exact same thing in TBC, and that resulted in the cave being moved. So, maybe that really would fix it again. But therein lies your honest answer: the map and the meta being played on it combined with queue times and their knock-on effects decide who wins and who loses AV. And yes, Alliance # of afkers vs Horde # of afkers is a factor in that. So is the queue time on Horde, which dissuades more casual Horde players from queuing but the same is not true on Alliance side. All of that factors in.
So, let’s say they move the cave and that works. We have some period of time, however long, where the win rate is, if not exactly 50/50, close enough to even that everyone’s happy with it. Then, the meta shifts again for whatever reason (TBC launches and the current honor system goes away for the currency system, or some bright guy on Alliance has an idea for a new strategy that really works and spreads like wildfire, or the increased winrate brings back more Alliance hardcore pvpers and your afk #s go down) and suddenly we shift the other way and it’s now very low winrate for Horde. Are they going to move the cave back to it’s current position? Doubtful.
Making a change to the map to try to address issues like this should be the last ditch effort because that will have a lot of unpredictable knock-on effects to the meta, and it’s a hard change to walk back if it doesn’t turn out the way you hoped. A map change should only be considered if the Alliance honestly cannot shift the meta the way Horde did. I’ll admit, I don’t know if that’s possible. It may not be, but I suspect that’s why Blizz rolled out the huge ban wave on AFKers (or at least part of it). They’re hoping that dissuading people from AFKing will shift the meta without changing the map, and maybe it will. If it doesn’t, they may be forced to change the map, but that opens a floodgate they don’t want opened: constant map edits to account for an ever-shifting player-driven meta.
In short, they’re going to hold off on making any map changes as long as possible, because they’re hoping we will fix the problem for them. And we might.