At risk of further flaming what appears to be a battle royale between Lemon and Lupus... I think you each have valid points in some areas. Here's my take on it currently, in appropriate order.
Destroyer mech wise your both right technically, Lupus is right in that Gamma islands need to be the source of obtaining them, as they would also likely be the head of an attack force on a heavily defended network of turrets. Lemon is obviously also correct in that the dev's obviously arn't anywhere near done with them. (As far as I know.) Same applies to artillery I'd suppose. And while rich people (Also me) would have access to them primarily at first, the same could have been said of when they added any of the heavy mechs. (They were UNGODLY expensive when they first became available.) Personally I'd strangle someone if it got destroyers in the game any faster.
Next topic, first off when you mention the alpha island market *** you don't mean me do you? (I assume not, as I haven't been playing for a while. And I don't recall ever seeing you when I last played.) Anyway, we can all agree that the upcoming changes are drastically changing the market and is causing hoarding of materials/bots. But this is rather normal, and should be alleviated shortly after the live patch. Will postponing also postpone the market issues? Of course, but I think that's a lesser evil than releasing a broken expansion. (I don't personally see the expansion as being "broken", just horribly balanced at the moment. Hell I expect it to be poorly balanced not matter what at live. It'll take time to iron it out with several additional patches.)
Next topic... Weak PBS's. I have to side with Lupus here. From what experience I have testing turrets over the last day or so on Gamma, it seems there would be a bit too much ease in raiding a PBS. (5000 armor turrets isn't exactly "defense tough". Granted they hit with some oomph.) I still need quite a bit more testing before I can make a reliable determination on exactly where balance needs to go, but I do get the feeling that PBS's need a lot more bang for their considerable buck so to speak. Cost to effect. At least defense wise, I think the facilities so far are fine. Also Lemon I think you and I talked about this in Trade chat the other day about the 3000+ active players and building it around that. Understand that doesn't exist yet, (I really wish it did, and I know the devs feeding their families do too.) so it's wise to build this at least somewhat around what the current playerbase can handle. (They could also add 300+ more gamma islands, but no one would likely ever even see 1/10 them with the current population.) Now it could be concieveable that if an entire alliance built a single PBS and used it you'd have around the clock people, individual corps just can't do this now, not yours, not his, and not mine. As such these PBS structures need to take a very large number of people, or at the least a moderate number of people fielding some expensive toys to tackle when even lightly defended by players. Granted there will be a fine line where you should be able to take only a few guys and whittle down a PBS when the defenders arn't online, but this should not by any means be a quick thing, it should a very long time to destroy a huge investment in time and resources with little risk in assets in doing so. Again this comes down to the good old, "Risk vs reward".
Adding a line break here because honestly it needs one so your eyes don't bleed. Continuing that last paragraph, if we go your way Lemon and make it so easy to walk into a PBS base or destroy the structures, your corp (Assuming your corp makes a PBS.) will suffer this effect and much as mine or anyone elses. One night all members in someone corp will gather around their timezone and walk into your PBS when 90-100% of your corp/alliance is offline. (Quite common with current pop.) And simply undo a months worth of work if not more. Yes they wont be able to take the actual base due to invuln, but they will remove all defenses, all facilities, all mining outposts, etc. To where even if you drive them off when you get back online, your still back where you started a month ago. Now all this in based on "current" base specs on the test server, I know I personally could wipe out a ton of structures in only a 4 hour window. (Yes terraforming defenses would slow this way down.) with a few of my combat alts and tank several towers with 2 RR alts supporting them easily. now this is "just me" let alone a whole corp doing this, or even just a whole squad. But enough on this topic, I'm making a wall'o'text here.
Last topic, the reward. Now again I agree with lupus mainly. There does need to be adaquite reward for such massive risk or if you prefer... massive investment in a Gamma island and PBS. Now we do get some as you pointed out, best facilities in the game. However note that with changes to the indy system, basic facilities will matter far far less than they do now. Though the CT combination facility is definitely a another benefit. You mention the epeen of obtaining one and slapping your corp's name on it. This doesn't in itself make having something worthwhile to many people, just as I personally find achievements in games to be utterly stupid and not worth my time. (Unless they provide a actual benefit to me.) Now some will love that, I take it from what you said that this does appeal to you, and that's great, in fact that means you gain something I simply can't. But you can't expect that to be the case for everyone.
Well I guess I did list two perfectly good benefits of a Gamma island, maybe it's just me but I think it needs something more for all that effort and risk of losing so much investment. (Yes as you can guess I'm all about gaining profit, that's my endgame along with fun pvp battles.) I "personally" would like to see Gamma materials used in destroyer mechs and only able to be built on Gamma islands. Thus making them high cost and high risk as someone could destroy the factory making them. (They should still be able to be shipped and used on alpha islands, to allow future budding corp/alliances to rise up and take over gamma islands that are being poorly defended.
I think Gamma islands NEED to be defended by players to repel attacks, turrets SHOULD NOT repel determined forces, however it should take a fairly large amount of time to destroy structures and turrets without bringing some major firepower. After all those structures cost a LOT to currently make (Especially the higher tier ones.) and at live will take a fair amount of time to build. They should be worth that cost in effectiveness, and so they don't insta kill attackers, that means they should have sizable amounts of armor to make them take time to destroy. (Unless you as an attacker bring a large investment thus taking a large risk yourself to make destroying them faster and more successful.
Christ that was one long post. I'd make a TLDR, but it'd likely end up being a paragraph long. Hope I covered everything, I probably missed a topic.