I'm not going to say mech is dead, but is mech still the only viable option now that we are 7 codexes into 5th edition? There are benefits of running an all mech army, and I still think mech is a good choice. On the other hand, I'm starting to think that it doesn't have to be the only choice.
There are some people who would like to say that there is no meta-game, but the state of the game does change with every codex. We started with an incomplete picture of the edition, but now things are much clearer. For most 4th ed codexes, it's hard to spam any long range shooting over a strength of 6. Anything stronger is over-costed or on platforms that are limited by FOC. With melta as the only reliable anti-tank in early 5th, rhinos and chimeras become very tough to kill from across the table. However, each successive codex has added more effective options for taking on light-armor at mid or long range. There is a lot of strength 7 and 8 spam now and most of it comes on foot options. There's now a way to build a solid long range fire base that is effective at taking out armor 11 and 12 vehicles reliably.
At this point I think that light armor is becoming a liability. To spam it effectively, you have to skimp on the units inside. 5 marines, even with a melta gun, are not very scary if they are stuck on the other side of the board. A good round of shooting from a newer 5th ed codex army can really hurt a light mech spam list. 10 or 12 Razorbacks are still tough to take out, but not as tough as it used to be.
Basically what I'm saying is that every army doesn't have to be min squads in razorbacks to be competitive anymore. There are tools to deal with them now, and variant list approaches for people willing to stop drinking the kool aid. I think people should look into foot or hybrid lists more. I'm not saying everyone should do this. The 4th ed codexes are still 4th ed codexes, but the newer codexes have a shot at pulling it off. I think it requires a different type of thinking than creating a mech spam list. You have to build the list around an idea other than getting the most fire power or the most bodies. It becomes more about selecting units that work together. Good Tyranid and Demon players do this, and I based my foot GK off the way the good demon players I know build and play their lists.