The only argument I can see against spin-offs is that the distribution of wealth in bitcoin is somehow "unfair." I don't think this view will garner much sympathy, however.
So you are saying that Bitcoin, a cryptocurrency whose users likely number in the hundreds of thousands
at most...
Are you saying that bitcoin's users likely number in the tens of thousands
at worst then? This sounds like hyperbole.
...is the be-all and end-all determinant of how any future cyptocurrencies should be distributed?
I believe it is the method the market will select, regardless of my perception of fairness. The more I study alt-coins, the less likely it seems to me that
any open-source and decentralized crytpocurrency could out-compete bitcoin or a bitcoin spin-off.
What would be the incentive for any new users who did not happen to hold Bitcoin?
A very credible argument can be made that a new user benefits most strongly by joining the dominant network. The largest network would tend to have the widest acceptance, greatest liquidity, largest market capitalization, and the strongest user base (allowing the new user to benefit as per Metcalfe's Law). If bitcoin tends to appreciate on average at a faster rate than a basket of alt-coins, this only gives more reasons for new users to prefer bitcoin.