If you lack velocity, you've got to have a different way to create open throwing lanes and windows. A Deshaun Watson doesn't "disprove" the value of velocity. He opens up throwing lanes and windows by being a rushing threat. This freezes/draws safeties, and allows receivers to operate in more open spaces. (It also puts him in physical jeopardy, which makes QB's that operate this way electric, but unreliable over the long run.) More open spaces necessitate less ball speed. Likewise, coaching and system can be used to create opportunities that don't require plus velocity. Montana operated in a Walsh WCO system that was unique at the time, and was full of quick slants, crosses, etc., that were essentially extensions of the running game. That system was far more about timing and accuracy than velocity, and Joe (like Brady, today) used the sheer frequency of these short throws to force defenses to commit to stopping the short game, which opened things up deep from time to time, allowing more leeway in the windows downfield. Take Brady today: you don't have to throw a rocket to get the ball to Gronk down the seam when the safeties are keyed on Edelman and Lewis coming out of the slot and backfield and Gronk is isolated with an overwhelmed LB thirty yards downfield. What velocity does is grant you flexibility in how you design an offense. It's absolutely necessary if your goal is to run a vertical passing game out of the pocket, unless your running game is absolutely dominant, a la 90's Cowboys. (Aikman didn't need a cannon to find Alvin Harper over the top -- he needed Emmitt, Larry Allen, Nate Newton, Erick Williams, et al to draw 8 man fronts reliably.) Most coaches do it the other way, because great OL's and running games are tough to build. They like a rocket armed QB and a target or two who can wreak havoc downfield so they can take the top off the D, opening things underneath for an otherwise less-than-stellar rush game. That last bit is what Cleveland will be hoping for out of their QB. The biggest open "window" on the field is often behind the defense. But that's true if, and only if, you have a QB who can sling, and a few targets who have the downfield skills to take advantage of that. To run that kind of O, you need velocity.