But a pre-mine where the developer slowly gets access to it over a 3 year period, that's only profitable if the coin is a success - the developer only profits if a lot of early adopters profit and continue to profit.
A tax per block does the same thing.
A Transaction fee tax has the greatest incentive as the rewards only come when people actively use the coin It forces the development team to do everything they can to encourage adoption.
One potential issue with a tax per block is that transactions that happen off the block in major web wallets wouldn't be subject to it.
e.g. when Mt. Gox was still around, a lot of transactions simply weren't in the blockchain because they went from one Mt. Gox account to another.
So people who did not use the major web wallets would be subsidizing those who did.