This “constant power” theory is wrong and you can easily see it by using the calculators on analytic cycling or by experience. You get more “bang for your buck” by using power on hills (i.e., 50 watts over threshold on a hill might be the difference between going 10 mph and 15 mph, where on the flats it might give you 22 vs. 21). The exact level above threshold is a dicey thing to determine, and you have to consider your recovery time, length of the race, etc.
And I suppose pushing 50 watts over FTP multiple times has relatively little cost (physiologically) to your race? Apparently you aren’t using the calculator correctly. What constraint are you applying? Obviously if you don’t apply a constraint then what you say is true but that’s hardly representative of what any of us can do in real life. Determining how hard you should ride the hills, flats and downhills actually isn’t that dicey since we have the NP paradigm/construct to guide us.
Thanks, Chris