I am not sure if Tim Cook is to blame for Apple's problems. Steve Jobs was a very hard act to follow.
I think the real problem is the culture and environment at Apple. They unfortunately learned a lot of bad customer relations habits when they were new with unique products. Now they either have to come up with yet another new and unique product or face the inevitable loss of market share to cheap knockoff products.
What Apple needs is a new innovative leader. I think Tim Cook has proven he can run the business. He just needs the next new thing and who knows what that will be.
Just my opinion.
I agree with EREBUS.
The fall of stock price may not have anything to do with the products. It might be a no-jobs-effect. I guess shareholders miss him. A successful corporation doesn't build on a person. It builds on the foundation of the company culture. The key industrial designer, Jonathan Ives, is still there. The key executives are still around. No one understands Apple better than Tim Cook. It would really be a surprise to me if a replacement will actually turn around the stock price in a reasonable short time.
As far as the Apple stock price goes, it was overvalued at over $700 per share and it is now undervalued at under $400 per share. The market often over or under corrects. Tim Cook and Apple as a company have not changed much in this interval. The business is still very healthy if not growing quite as fast as before. I don't think that Apple will ever get back to the unsustainably high growth that they had regardless of who is running Apple. I would argue that Apple would have followed a similar trajectory even if Steve Jobs was still in charge.