Or is it just the rise of the Xenos(and other armies) that have contributed to the supposed decline of IG. First off I don't think the IG have declined at all, the army is still great and more than competitive on its own, even more so with allies. I think there is a few things going on here that are contributing to this "decline" though. First is simply that other armies have gotten better, across the board, all the new codex's are competitive, some more so than others (Tau). What this means is that IG is no longer king of the hill with all the undesirables beneath it. Now we have to share the table and sharing sucks. I personally have had to work a little bit harder to win recently, and I like it. Are the uber net builds hard to beat, yeah and you will struggle against it. But most people don't play that way, or even have the models to do those builds.
I am personally of the opinion that what you do on the board(tactics) is more important than the models that you bring(list building). Don't get me wrong, you can certainly build a crappy army that couldn't win if your opponent rolled 1s all night. But I have seen people get their asses handed to them even with one of the super powerful builds, because even as powerful as they are, this game is far more than just rolling dice.
Its easy to get all doom and gloom. We have had good codex releases in rapid succession, giving you little time to adapt to the new armies before something new shows up. Now I want you to think about this, I remember back when farsight was released and there was all the commotion about the farsight bomb and how OP it was. Now fast forward, how many of you out there are facing this all the time from multiple Tau opponents. The answer is your not, like anything, it had its time in the sun, people adapted and countered it and most moved on to the next power build. Most that are left playing it are the ones that just like it personally.
What this has meant though is that you have to be constantly adapting and trying to figure out how to counter these new armies, but now have little time to do it. By the time you have figured it out you have to contend with the new one and start all over. Change is hard, but get used to it and adapt, just because the build you have been using for a long time no longer works, does not mean there are other ones that are just as effective. Adapt, overcome.
Now a lot of people point to the tournaments and the "meta" to support the assertion that the IG have fallen. Well first off, how many of those people that are playing tau and eldar and daemons now were playing IG a year ago? Most of us have multiple armies, and the tournament scene, especially the national stage, is a relatively small sample of 40K players in general. A lot of people in tournaments jump around armies to stay competitive, in their minds, so this can greatly skew the numbers and give an impression that isn't necessarily true.
I see IG winning all the time, and I personally have not lost a game in quite a while. To be fair I have not had a rigorous gaming schedule so that has something to do with that ;). Have I had tougher games, most definitely. And I have still yet to face the screamer star, so far no one in my gaming group or at the FLGS run it so that is that. The great thing is now you don't have to play a game to figure out how to beat an army, the wonderful interwebs has done that for you. You will still need to play though, that is the only real way to improve and as much as I love to theory hammer, at some point you have to roll the dice.
Now where does all this leave guard players? Well you have to be flexible and unfortunately it comes down to having the models, and as IG that can be hard with so many choices and in needing so many. I didn't play with vendettas for a good while, I didn't have the hulls or lascannons, but I have been playing with them for a while now, and most of my armies has one or two. Also don't let opponents guilt you because you are using a good unit, everyone uses their good units, that's how it works. Now if you are that a hole that brings 9 vendettas well, you may deserve those stares you are getting, sorry. And to be honest you don't need 9 vendettas to win, nor is that fluffy, an air wing would have a mix of units to deal with different threats, not just all the anti tank.
So adapt, change your tactics, and you may have to buy a few new models, that's how this game works. Sure it would be nice to find that "perfect" list and just be done and enjoy winning forever. That's never going to happen. So endure, have fun, and be thinking ahead. Tyranids are coming, your going to have to fight them more now. How are you going to incorporate that into your army and still be able to deal with Tau and eldar and daemons and space marines and chaos etc. You see, you are going to always have to change, but always have to deal with the same people as well. Your tactics will be different against someone you play a lot vs someone new (he doesn't know that neat trick you do).
The good news is you only have to endure one more codex before the time of greatness. The IG codex will be out soon, and after Tyranids there will be a good break before our enemies are strengthened again. Things are going to change with our new codex, and the way you play is going to change greatly and your current lists are not going to work as well. That's ok, don't bemoan that fact, instead embrace the new options and kill the xenos in new and exciting ways. IG could get a great update or merely a meh one, and you know what, as long as the nerf bat isn't huge, the army does great as it is and some points changes and unit moves aren't going to change that. Some units are going to be useless and some useless one will be playable again (ogryns).
We will see the meta shift again like it has with every new release, some one will figure something awesome out and everyone will use it for a while till something new and shiny and move on.
Just don't worry that guard has fallen, we haven't, its just crowded at the top these days.