Midlo Dad,
The diagram is OK, but the numbers don't work out. For example, if a radar gun measures the speed out of the pitcher's hand as 90mph, and 82 mph at the plate, to explain that as the effect of the increased downward angle of flight would require that the ball be traveling downwards at 24 degrees from horizontal. That's .45 feet downward for each foot of horizontal travel, which is absurd for a fastball. After all, some peole think (wrongly of course) that a well thrown fastball rises as it crosses the plate!
And, as has been pointed out, there are plenty of Pitch F/X measurements which aren't susceptible to cosine errors that show about the same loss of velocity as a radar gun.
Here's another way to look at this. A half century ago, Lyman Briggs used a vertical wind tunnel to measure the drag on a baseball. He found that the upward wind velocity required to suspend a baseball is between 90 and 100mph, which is conveniently close to pitched ball speeds. So the force of wind resistance on a collegiate or pro fastball is very similar to the force of gravity. That implies the deceleration of a good fastball is quite similar to the acceleration due to gravity-- about 32feet/sec/sec. Works out to 8.5mph for a 90mph fastball-- something of an overestimate because the drag decreases with velocity.