so my theory has always been that with the technology available, why can't you just measure the last 20 yards, or give everyone a 20 yard head start and measure a top speed 40 yard dash.
That would be more relevant to football speed. They already measure agility in other ways, and no football player ever starts from stand still in a sprinter's position and takes off for top speed.
Even WRs, don't get to top speed from a stand still. Even on deep routes, they have double moves and things of that nature before they hit top speed.
For any player, the top speed is only going to matter in the open field, and you don't get to the open field immediately.
A true top speed measure would be better, combined with the agility measures.