You're missing the point.
Velocity's value these days is mostly increasing the range between pitches, if like Irvin you struggle to throw 90, it compresses the velocity difference between your FB, slider, change and curve and allows the hitter an extra tenth of a second to recognize what you're throwing.
However, ML hitters can routinely hit 95, even 100 MPH FBs down the middle of the plate, self-selection have eliminated most hitters with "slow bats." Especially if said FB has little movement.
So while you want to draft guys with the potential to throw hard without max effort (b/c max effort guys tend to have worse command and make it harder to throw off speed pitches without a "tell"), once you get in the mid-90s, additional velocity has limited marginal value. So you shouldn't obsess over velocity v other, more important aspects of pitching once you get to the velocity threshold (these days what we see with Nola and Suarez, average 92-93, t95).
Nola's struggles aren't due to a drop in velocity like Arrieta, but his failure to command his 2 seamer, and his erratic curve and changeup. When he's "on," he paints the corners.
Also, distinguish between velocity and stamina, there are pitchers who can throw hard through 100 pitches, but tire and start losing command before they lose velocity. When pitchers start missing spots and run deep counts, that's usually the sign to get the bullpen up, because that often precedes the drop in velocity.