Steve Jobs had a certain personality and character that he could be hands-on -- there is no-one at Apple who has that. SJ also had charisma, and I'm sure some of his teams would have been willing to make big sacrifices for him and Apple. Forstall doesn't have that and can't emulate SJ. Look at all the Keynotes where Forstall was asked to demo features -- I always thought he was far from relaxed, not a natural in front of his peers or journalists and certainly rubbed-me off the wrong way.
Also, Senior VPs don't do the design work themselves, they provide ideas, play devil's advocate and provide direction. Each SVP has a team of very talented people who do all the hard work -- after all, Jonny Ive isn't going to be designing and drawing icons for MacOS or iOS or doing layouts and functionality for Apps.