He's forty now so obviously he doesn't have the same muscle tone or recovery period from exhertion as he used to, but it's not really a massive drop off by any metric. The truth of the matter is that Drew Brees never had a powerful arm. Not by NFL standards. It was even a ding against him coming into the draft. He is the Greg Maddux of the NFL in that his performance level has always far surpassed his talent level.
Drew Brees' arm strength coming out (of Perdue) was below average at best
Quarterback Ball Velocity at NFL Combine 2008-2017
For the record, the only velocity number I can find for Brees is 54.7, placing him squarely in the "below average arm strength" category for essentially his entire career.
Weak arm and short stature means that Brees gets any power or velocity from his core. Not surprising that he concentrates on that very aspect of his game when training.
At 38 Years Old, Drew Brees Believes His Best Football is Still Ahead of Him
Brees' arm strength will forever be tied to his mechanics. Suggesting anything else is simply ignoring what we have known about him since college. For the record, I'm not a "Brees Forever" person either. He is at the very end of his career, and will, if he doesn't retire first, exhibit a marked decline in his on field performance. But I will always question when it is attributed to any perceived loss of arm strength, because he's never had any to speak of...