The argument about specialization being less common is backwards. In Pathfinder it should be extremely hard for someone to get the support needed to focus all day on learning/studying/experimenting on magic, only the rich or those accepted to a few magic schools for those with talent could really do so since most people would have to spend all their time supporting themselves and access to knowledge would be rare. In a modern/advanced setting mass production of necessities means far more people can afford to specialize in certain fields. Combine this with easy access to knowledge and there is no reason why people wouldn't regularly become high level casters. Some groups might want it kept secret but that isn't easy when once data has gone digital.
I also have to agree with the no arcane, divine, esoteric thing being slightly problematic. Wizards/Arcanists seem to try and discover new spells, divine caster can literally ask their god, Psychics don't have much in terms of tradition so you think some of them might have tried to replicate spells from other classes. This doesn't even begin to address the classes that used magic in non-traditional ways like the Medium. Thats not to say that there can't be ways to get magic A to do what people in Pathfinder thought could only be done by Magic B, but the book presents the mystic as being able to do this when really it can't quite do all of Magic A or B and PF had more than two types of Magic user. It creates a disconnect between how magic has supposedly improved and what spell-casters as a whole can accomplish which is a lot less.
I think all of this could be fixed by presenting a mystic as someone who regardless of power source sacrificed specialization for versatility. That, however, would bring back balance issues which is really what this boils down to. My best explanation would be that while there are more 9th level casters than ever before, the presence of large organized governments and corporations generally interested in keeping the status quo (or controlling changes) means 9th level casters are either carefully watched by groups with a lot of power or keep their heads down to avoid the notice of those groups and aren't throwing around high level spells whenever. This creates an explanation for why an adventurer might be hesitant to learn spells if the result is ending up in a mandatory day job under 24/7 surveillance, by groups capable of mobilizing the resources needed to deal with even wizards of that power, after all some people would take the offer.