I just feel that any motive other than profit, will not be good enough. I agree that profit is not the ONLY motive for generation as you have pointed out, but other incentives are just too weak and will result in too few generators. The network will be too weak and will surely fail.
Not surely, but maybe. I don't think so because of the preservation motive is nearly as strong as the profit motive, and because there will still be some who can generate at a negligible additional cost, so there is no point at which generation is unprofitable. The confluence of these motivations will ultimately create a self regulating market with a fairly stable difficulty level. The real question then becomes, is that difficulty level secure enough to defend against a brute force attack? That is a question that may never be answerable.
How high do you think the difficulty would be now, if there was not the 50BTC incentive to generate? I think it would be < 1000.
That's the wrong question, because none of us pay our electric bills in bitcoin just yet. The question should be, how high would the difficulty level be if the reward were worth less than $13? The answer would be less, but the total value of breaking the blockchain would be less as well. It's a self-balancing system, almost organic in how it acts; and any risk calculations that could ever be devised would be incomplete due to the human factor.
No one has commented on my idea of placing a rule on fee distribution on blocks to prevent the acceptance of arbitrarily low fees.
This forces clients to compete with each other for the current block, eliminates the need for a fixed block size (ie. fixes the spam problem) and scales to accommodate any number of transactions. If a spammer sends lots of small-to-zero fee transactions, only some of them, if any, will be accepted, because you can only include max 50% fees which are below the average. To include more smaller fees a generator will have to include more larger fees.
Think about it, clients will compete and a stable market rate for fees will be established. Tiny fees, too far below the market rate will not be accepted soon and fees above the market rate will be guaranteed to be accepted in the current block. It's a simple protocol rule for generators which should be computationally cheap.
I was still considering it, myself. I think that the priority system does much of what you are trying to do
except allow the blocksize to stretch; and I have concerns about the bounds of this rule. Without the priority system, this rule would permit blocksize spamming anytime there were only free transactions in the queue, so there would have to be a minimum allowed average fee, even if that were just .00000001 bitcoins, but then that would prohibit any transactions from being processed at all in the absence of a fee paying transaction even if there was an altruistic generator who won the block. So there would have to additional rules to permit altruistic generators to help clear transaction backlogs, because there is bound to be more free transactions to process than fee paying ones for the foreseeable future.
I offered a plan before you posted yours, and no one commented upon it either.