I think the term you used 'marginal entrepreneur' is appropriate if the tax rate is a significant factor in going forward with a business.
As you said, if the tax rate was in some ridiculous extreme, of course that would be a factor. But then I'd wonder what kind of country/economy would raise taxes to such a high altitude.
In my old line of work, we whipped up a spreadsheet model to show a new company's potential return on investment before we took the idea to investors. For any given company, there is a tax rate high enough that would have made it non-worthwhile.
Of course, we weren't in the web business, so numbers mattered to our investors.
As you said, if the tax rate was in some ridiculous extreme, of course that would be a factor. But then I'd wonder what kind of country/economy would raise taxes to such a high altitude.