There is no algorithm you can't build dedicated hardware for, except one that changes frequently and unpredictably. But can you trust or rely on something that changes frequently and unpredictably? Is how it changes inherent to the protocol, or based on the whim of a central authority?
How about an algorithm, that (currently) relies on specific features of specific hardware? For example, the current generation of Intel or AMD processors for desktops or servers, the amount of cache or RAM they have or must have, and certain instruction sets built in to those chips, such as AES or SS-something.
It will last for a few years or a couple of generations, but ,.. eventually GPUs will have more power than CPUs, and FPGAs will get built, and then ASICs.
In the short term, you may have a coin that can only be CPU mined, because whatever algo it uses is best run on a particular architecture, such as the latest Xeons or Core i7s, because it simply won't run on a GPU that doesn't have the same amount of L1, L2 or L3 cache and RAM. The problem then is that to mine this coin would require a computer where you reserve that much RAM for it, or possibly even dedicate the whole machine to it.
I can see desktops today that have 8 GB to 16 GB of RAM on them. And I see servers with 36 core processors and 2 TB of RAM, where you could virtualize your miners to run a whole bunch of them.
I don't see GPUs with that much RAM. I definitely don't see FPGAs or ASICs with that much RAM.
It's much better or more practical to just get that particular hardware.
Even in this case, centralization will happen, because someone will just buy a whole bunch of machines and stuff them in a warehouse. Yes, everyone that can afford a desktop unit can be a miner, but the big guys will do the same thing. And there will always be pools.
I don't know how mining is going to turn out for bitcoin, but at the very least I can see the technology can exist way into the future and usage of the coin grow.
As for you guys with large warehouses and cheap power, you may or may not help the little guys hosting their stuff, but for sure, it would make sense to you to also run your own stuff, because eventually the little guys will either pull out or just stop.
But don't underestimate the hundreds or thousands of altruistic miners. They're the same guys looking for digits of pi, looking for prime numbers, looking for ruler thingies, cracking RSA, looking for aliens in outer space, finding a cure for cancer, and a bunch of other things, all for no pay.
And don't forget the small pockets of "rogue" or opportunistic miners, people who stick their little boxes in their office (for free power) or dorm rooms (for students).
As long as a manufacturer stays in the game and sells end products to consumers, someone will buy them and run them. It's a pity that this thread's manufacturer is gone, they had some good miners during their time.