Why would you call emission rate a tax?

When the block reward was 50 coins the inflation was higher than 10% per year, it gets lower and lower with every new halving. Bitcoin was designed to distribute coins over the course of several decades on a permission-less basis. All coins (before they are mined) are guarded by an algorithm, as opposed to any form of private money (including fiat) guarded by a group of people.

It might be cheaper to maintain an electronic money system, where people are at the center of its security model, but you will get all the downsides of that as well.