And the same logic to Deng and Gay applies. If they were young players, you'd be re edly overrating them, as well.
No, the SF position has not gotten significantly better. You forget how good the players you mentioned were because the majority of them were derailed by injuries or past their primes. With the exception of Kawhi, Bron and KD still rule the position, like they have since '10. And

Hayward, Giannis, Gallo, Crowder, Stanley Johnson any better than Gay, G-Force, Granger, Deng, Artest. Wiggins will be one to watch, perhaps. But he might not wind up any better than Gerald Wallace, who had similar athleticism and skillset.
The only reason the SF position is "somewhat" better is because the emergence of Kawhi and to an extent Paul George, but overall, it's not that much deeper than it was in '09-'12.
Jimmy Butler is also an SG.