Except, all the evidence suggests Tim Cook is doing exceptionally well, and Apples inhouse adverts are driving record sales amongst their products.
So your opinion is, in fact, easily falsifiable.
The world and Apple is in a very different place right now than when Steve left it. Android phones are solid competitors to the iPhone, and the industry is more competitive than ever. Each passing year people expect ever further levels of perfection from every aspect of their phone. So this compounds into two fracture points, 1) You have to do more and do it quicker to stay relevant, and 2) The better things get, the harder it is to improve them. It is easy to look back and say everything was better under Steve, but was it really? The iPad launched with a near identical UI to the iPhone with little to nothing in the way of additional features, this was bad. The iPhone took multiple generations to finally get a good camera, this was also bad. How about antennagate ? this is remarkably *big* mistake that Steve as CEO ultimately was responsible for. We have not seen anything on that scale of 'fail' under Tim Cook. Tim has year after year delivered market leading products, with incredible consumer satisfaction and ever growing sales. We have amazing tech like Face ID that just a few years ago sat in something twice the size of a phone (see Kinect). The guy deserves more credit than people give him.
Steve was a great guy and a prolific innovator, but he was not perfect, and nor was the Apple he commanded.