If, by then, many people have quit, how would the difficulty continue to rise?
The future of mining is in FPGAs. CPU mining was replaced by GPUs. GPUs will be replaced by FPGAs. FPGAs will be replaced by ASICs. The latter will only happen if bitcoins manage to keep their value long term and more real world uses are found for bitcoins in general. ASICs require a very large investment.
I disagree regarding FPGAs. They are insanely expensive for the MH/s, and if you're only looking at the difference in electric costs, it would take years to pay back the difference. Not to mention, they're not going to be as easy to resell... Very few people will make significant investments in FPGAs. The only people who will mine on FPGAs are those who do not care if they are wasting money, or those who already have access to clusters. It's too risky of an investment otherwise, counting on being able to mine for years and years.
ASICs might have possibilities, but again, it would require significant up front capital that would take months or years to pay off. But, I could definitely see someone interested in technology who just might give it a try. They make a whole heck of a lot more sense than FPGAs, at least if the estimated performance figures are in the ballpark.
I remember the times when people (and me as well) distributed some calculation power for SETI@Home, BOINC, Folding@Home, or wanted to be the number one in a benchmark rating chart. This all for free and never got some money back for the wasted energy, invested time, patience, and for the whole equipment used for.
Now, with Bitcoins you get a little bit back for using a computer, and the opportunity to win a lottery is also much fun.
There will be always people who want to participate in a race.
Especially when graphic cards are rare for miners then prices increase.
When Bitcoins becoming seldom the same will happen.
This is true, but it is a matter of how many people you can find who will mine bitcoins at a loss. It's not a "feel good" project for most people. For most miners, it's all about the money, and they will leave when the money leaves. Sure, there will always be a core group of people who will mine no matter what, but when you're looking at the overall difficulty level, the vast majority of those megahashes comprising the difficulty level will be out the door as soon as it becomes unprofitable. Which means, it will become profitable again.
Like I said, the only way we'll see close margins for some of the better MH/s/watt video cards is if bitcoins are reduced in value (somewhere on the order of 1/5th of what they are currently worth), or if they somehow, miraculously, stabilize at the exact same value for the next several years.