Yeah that looks quite effective. Willier have something vaguely similar with an elastomer at the seat stay / seat tube junction (see link below). But for road riding I find that 28-32 mm tyres and a D-shaped relatively compliant carbon seatpost, along with relatively thin carbon seatstays, provide plenty of comfort. It's usually the front end I sometimes find a little harsh. Carbon bars help a fair bit with the vibration and most carbon forks allow some degree of compliance. But I can see why the Roubaix exists for riding cobbles. For riding rough gravel the Cannondale system looks pretty good with 30 mm of compliance. I'm a little surprised they didn't match it with a front suspension, other than a compliant fork. I don't recall many XC bikes having a rigid fork + rear suspension. Quite the opposite!

Edit: One thing about the Cannondale Kingpin I would be concerned about with 30 mm of compliance is damping as it appears to be an undamped spring and that is quite a lot of movement. I used to have a Cannondale Scalpel, which also used flexible chainstays for rear suspension, but it also had a spring/damper at the seatpost junction. The first version of Specialized's Futureshock was also undamped and some people found it too "bouncy". The Mk2 version added a damper, which was considered a significant improvement. But maybe the damping is not so critical at the rear end. The Willier system I mentioned will have a fair amount of natural damping in the elastomer.

https://wilier.com/en/my2022/int/rac...kes/cento10ndr