Sub 4.5 is Elite. He doesn't need to get that. Wasn't Butler like a 4.48 at the combine, and Lazard around a 4.61? I'm guessing Hutchinson is in the 4.6 range too.
I think I agree with your premise. First let's say that having speed is preferable to not. The headline number of a great 40-time carries some weight, probably too much. However I think it's been shown to have little correlation to success in football at a particular position, even positions where speed is at a premium. You could say there is a minimum required speed, and beyond that is either a bonus or a useless metric. Some fast guys were terrible WR or RB; some guys running a 4.6 are HOF material.
What is more critical for a WR, especially a NFL WR, is being able to create separation on his own merit. Speed helps, but is not the only important variable. Some of this is route technique, coverage awareness, short-area quickness, getting in and out of breaks smoothly and quickly, etc. Plus, of course, being able to catch the ball. Many of these things can be significantly improved with physical training and technical coaching.
This is why it's rare for freshman or rookie WR in college or the NFL to have huge seasons, because there's more to the recipe than athletic ability.