Herrera is considerably above average even with his streakiness but if we've seen his ceiling his value not great going forward, especially if he has to move off centerfield. The hope was he would become an all-star and a batting champion (as per CD), which he was for two months this year. But failing to do that doesn't mean he's not still a very good line-up piece. Or that he's easily replaced.
I don't think the Phillies are going to be complacent/deluded about the young players at all. They're going to be super active on the trade and free agent market. Just look at the deadline. 25% of the starting 8, replaced without a thought. Plus a starting-caliber 1B for the bench who may give them different options going forward. This is what they've been waiting for and they have pieces to deal.
I think that's right that nobody besides Hoskins, who is miscast in LF, is a sure thing to be here in two years (or even next year). And Herrera and Kingery's contracts make them more tradeable, not less tradeable (though Kingery has to do more first).
Defensively, I think it's true: there is nobody on the current team who is going to become better defensively except Alfaro and Crawford/Kingery who we haven't even seen yet (in their correct alignment). We can watch Franco make good plays but he's never going to be great (and our perception of his "improved" defense is tied to his bat, just like so many Golden Gloves are). Ditto Williams and Hoskins. Santana is aging, Hernandez is what he is. Herrera can return to form but he's aging too. And our two best defensive OFs are injury-prone and unproven (or in Altherr's case, almost out of hope).
That said, the team's success is breeding impatience. Understandably, of course they should seize the brass ring. But it seems obvious every other team in the NL is flawed too.