Lot of different takes here. I actually agree with Thiem (based on my viewing of their US QF match).
The aggression in shot selection is higher in these two than it was/is compared to Djokovic and probably Rafa and Federer too. Perhaps 05/06 Fed before conditions slowed he looked to attack with his forehand to a degree similar to alcaraz.
Doesn't mean the weight of shot or speed etc. is higher. Certainly not.
While Federer's court position was more aggressive, he played with more calculation. Sinner and Alcaraz are young and fearless and they are still exploring their limits, so in a sense it's natural to see young guys go for the kill early (07/08 Djokovic was hyper-aggressive, as was an 05 Nadal on hard). Alcaraz especially makes a lot of errors trying to belt the cover off the ball from difficult positions.
We also see far more drop shots from the Alcaraz/Rune/Sinner gen -- and this is a form of aggression. It's a risky shot.
In short, the calculated baseline rallies of attrition that were a hallmark of Nadal/Djokovic are not evident in these young guys (yet—maybe it will come, but my instinct says no). Sinner and Alcaraz look to take risk earlier in the rallier, whether that means changing direction, drop shot, coming forward (Alcaraz) etc. They do so more than the big-3 when given the same ball.
*Also: But I don't think this is an 'evolution' of the game. It is just a shift in style. We see more winners but far more errors. The Big-3 are still the peak of modern tennis by a margin.