Furthermore, there are trumpets out there that have a greater taper than many cornets, and vice-versa.
And the conical tubing angle is meaningless if you're dealing with Cornopeans and old-model cornets that have more cylindrical tubing than either the modern trumpet or cornet. The instruments in question consist of sections of stepped tubing, which serves the same purpose as conical. Think about that a moment: cylindrical tubing that serves the same function as conical, in terms of sound.
This means that you can't, and never could, point to the slope of the conical tubing as being a meaningful factor.
I wish Cornopeans were more plentiful these days. They still turn up from time to time on eBay. Many of them have a great old-fashioned cornet sound. Yet they're roughly 50-50 in terms of conical vs cylindrical tubing, and the so-called rules of what makes for a cornet don't apply to them, any more than they apply to the old S-configured cornets, with mostly straight tubing.
Look at the Cleveland cornet at the top of the page: