- Ease of Acquiring
This is a big deal right now. If you weren't already in the game and have some BTC, you cannot currently easily buy an ASIC miner. Up until this point, mining was driven by the ability for someone technically capable to go online and spend fiat to either buy another CPU, or buy more GPUs and have them running within a week or two. This is no longer the case, and until ASICs are put out by a few companies, shipping within a week, and selling for fiat - there will be a significant amount of miners who have a vested interest in keeping GPU mining relevant.
Yes, all the present GPU miners would love to keep GPU mining relevant but that is not going to happen just because they want it. There have already been enough ASICs pre-ordered and on their way to kill GPU mining for good. I listed them in moy other thread, but you have 2 batches of Avalon miners, plus hundreds of thousands of discrete chips, all the BFL pre-orders, ASICMiner USB and Blade sales, plus the 200TH ASICMiner is adding this summer. That is ASICs that are already coming. They were ordered long ago and just beginning to ship now. Even if a single new ASIC was never sold from here on out, GPU mining would still be dead. I don't think GPU mining can survive a couple hundred million difficulty and that is what we are going to see soon.
GPU mining can only live due to litecoin. But I believe litecoin will die sooner than later because it is purely used for speculation. You can't spend it anywhere and it offers 0 advantages over bitcoin. It's going to be a market of litecoin miners selling litecoins to each other. No one else buys them.
When ASICs are cheap and easy to buy within a week they won't be profitable anymore. The margins will be so low that only people with free electricity will even bother. Competition will drive the price down so they just barely make their money back, I'm guessing in a year or two. It will be a low margin grind and won't be much fun except for those who really enjoy mining for it's own sake.