This isn't how real outbreaks progress. Things will change over time. Your DTR won't be constant. A real projection would have to model transmission rates as the disease moves into new territories and populations. But of course we won't really know how that's going to work until it happens.
You're right, of course. And I've tried to make that exact point in different ways throughout this thread.
Nevertheless, the point of doing the math for those who won't do it themselves is to show the pooh-poohers that this really is something to be concerned about. Sooner, rather than later.
I've wanted to show the growth of the epidemic, based on actual data, not on vague statements we're hearing from the government such a "the epidemic will get out of control if we don't do something soon." Because that's all we get. And when they do provide projections, it's only until the end of September. Based on current rates, the real action starts in December or January. The numbers help people see that for themselves.
If they see in real numbers where this is heading in the near future, perhaps some people will take heed.