One of the primary purposes is to shut down the existence of HFT as we have established that it is parasitical - at least, my arguments as to why it is parasitical have yet to be countered based on anything derived from the tangible universe in which we live, but I eagerly await any angle on this which I have yet to consider. There are a few ways I interpret the latter part of your post: that the big HFT banks would be able to continue with HFT? Or that big non-HFT operations would simply squeeze the little guys out of the market?
Additionally, while I hate the financial services industry as much as you do, we both acknowledge that it IS a large part of the economy. This tax would decimate it, and what becomes of all the millions of people that work in that industry? The tax will make them jobless overnight, but there won't suddenly be a glut of manufacturing jobs for them to jump into in the same time frame.
Reminds me of the argument that if you eliminated the income tax then all the tax preparation services would go out of business. To me, this is as near a perfect example of the 'broken window theory' you can get in practice. There is a large group of 100,000 or more people probably involved in preparing taxes and writing tax prep software. If we eliminated this need or greatly reduced this need by only passing income taxes past a certain threshold then they would be out of a job, no doubt. But they are already employed whatsoever from the government largess doing work with 0 productive capacity, scant justification for social necessity, much of which basically involves trying their hardest to defraud the treasury by a maximum degree of tax avoidance, etc. We must ask ourselves do we allow for the slow drain on our economy by fearful indecision in the face of the clearly wasteful enterprise that is solely propped up by government in the first place? I would argue not. Much of this argument applies to the financial services industry.
Our financial services industry needs to shrink. It has been growing at the expense, not at the benefit of the 'real economy'. It made 40% of corporate profits in the past few years. I've heard statistics that it is roughly 17% of our entire economy now. This is much too high and has only been allowed to exist through: the unsustainable financial deregulation that has been unmitigated disaster, nearly 0% interest showered on the too-big-to-fail banks, bailouts, regulatory capture, quantitative easing etc.
In my system though, there would be job-retraining programs and massive public works and research programs as there is no shortage of these things that need to be done.
I am much for ending the global empire too, but not in favor of dumping a million solders on the streets with no path for rejoining the workforce.