I'd say that injuries to pitchers are no less frequent than they were 40-50 years ago, but that they're treated differently. Shoulder tendonitis, frayed labrum, torn rotator cuff, and on and on. Perform the surgery, rehab the pitcher and bring him back next year. Fifty years ago, there was only one injury to pitchers and it usually meant the end of his career and you never heard of him again: sore arm.
Oh, what could have been.
http://tinyurl.com/2dl6wc Note the reference to the injury that ended Spooner's career. Spooner was one of many.
Pitching is also much more difficult today than 50 years ago. Ballplayers today, making millions of dollars, have workout rooms in their homes and work year round to stay in shape, continuing the process in the state of the art facilities that modern ballparks feature. Back in the day, ballplayers worked second jobs in the winter, drank beer, and showed up fat and out of shape for spring training, which nowadays is somewhat of an anachronism in its length. Pitchers, therefore, work much harder from pitch 1 to pitch 100 than pitchers in the olden days.
(All of the above, of course, is wrong. Eastender will be pleased to state that it so, followed by an arcane explanation of something having nothing whatsoever to do with what I just wrote. Sigh.)