First, who says that he can't?

Second, who says that his vocal decline is caused by stress only?

I feel like this myth deserves to be debunked. He already showed big signs of decline in early 2013 before Richie's departure and, as you said, the decline was subtle. He didn't lose his voice right away, he lost it gradually over the following months.

I'm not saying that stress couldn't play a part, but I refuse to believe that is what caused it. Any trained vocal coach will tell you that you should sing from your diaphragm and not straining through your throat, which is exactly what Jon has done for his entire career.

Stop looking for excuses and accept the fact that our beloved Jon did not bother to develop a sustainable singing technique and is paying the price.