Be realistic, walls wern't the issue with PVP, it has been and always will be the server population. Why walls came to the forfront is simply that's where a lot of players are, behind them. This will only get magnified with gamma if the population doesn't expand to meet the new areas.
If you look at the posts about PBS, a prevalent theme is that these destructables are going to 'force' players to undock to protect them. The concept here is that small roaming groups will be able to visit these gamma islands and get fights whenever they want, or at least have something to blow up to make the trip out there worth it.
The reality though, is you can't force anything, either the defenders have enough players/bots to counter the roam - or they don't. Because of the destructable nature, gamma builders will use every tool, walls, TF, turrets, to prevent roaming groups from getting anywhere near the destructable items. Stopping roaming groups from blowing up desctuctables, by extension means, they are also not getting small group PVP on gamma either.
At first glance, this is OK, since beta is there for small group PVP still. Except, there has to be players on beta for that to work. There will be a number of alpha players/corps that will sense an opportunity to move out and use beta, hoping that a power vaccum has occured because vetern corps have moved to gamma. And this will probably be true for a short period of time while gamma corps build up defenses. But once they are done, they will quickly be bored, and be roaming again; assuming again that gamma defenses prevent small group pvp, then they will head to beta.
Roamers will go to where the targets are on beta. Making those areas 'unsafe' again, which will either drive those players back to alpha, or out of the game.
Again, this is based on current population sizes, simply meaning there will not be enough targets to satisfy all the roaming, so the small target base will get over roamed, frustrated, and leave.
If however, gamma defenses do not provide enough protection, corps will find the continual loss of PBS unsustainable. The nature of this game predicts those driven from gamma will pressure other gamma outposts in the same way; if there is only 1 gamma post, that's where the players will be, so that is what will get roamed.
tl;dr - A proper balance between defense and player base needs to be monitored and adjusted, there is no 'one-size-fits-all' solution. This is the greatest challenge the devs have, making the game challenging but still enjoyable.