Firing Scott Forstall was Cook's biggest mistake and it has negatively impacted the quality of Apple's software after he was let go.
I'm not sure if that is really true.
As far as the public is concerned, he was fired over the "Maps-debacle".
True, he might have had a copy of a memo to Tim tucked away somewhere where he asks for more funds, more people, more time to "make it great". But IF that is the case, he bet too much on such a get-out-of-jail free card.
When Jobs overplayed his cards, he had to leave, too.
Based on some of the recent products we've seen such as: iPad Pro/Apple Pencil, Apple Watch, iPhone 6 Plus and the Retina Macbook not too mention a bunch of the new accessories, Tim Cook comes across as a complete yes man in regards to giving new products his ultimate blessing.
That is just an assumption.
Though that mouse-charger leaves a bit a sour taste, I admit.
That said, a company like Apple (being nothing resembling the Apple that Steve Jobs returned to in the 90s) would be incredibly difficult to lead, simply due to its size and the kind of persons needs to attract to actually keep it alive.
Steve did a hell of a job on that - but I'm pretty certain we all know that it couldn't have gone on like it used to go for much longer anyway. Illness or not.
Tim Cook is at the moment the right guy for this job (as much as I can see from my armchair here).
Not sure if Forstall - him being a bit too hard-trying in imitating his mentor for my taste - would actually excel at the job. A job, he apparently was rooting to get.
At least, Tim is Tim. He's no Jobs and doesn't lose a minute trying to be.