It seems to me that guys like this are a big exception, maybe there are a ton more notable top 4 level d-men who didn't produce in juniors and I just didn't notice, but I've seen a few places that have more or less correlated points in juniors and NHL-level defensemen pretty well. I think not scoring in juniors is a big red flag that you don't have the skills to even play as a defensive d-man in the NHL. Maybe both those guys developed dramatically at a later date, but I always regarded someone like Carlo as a unicorn.
I think at the end of the day, it's possible to teach a defenseman to play more responsible defense, but players who couldn't produce offense at lower levels will not develop that skill 99% of the time later on. Obviously there's a cap to how good someone can get defensively, but it's clearly the side of the ice that can be taught later, with offense you either have it or you don't. It's something that can be developed, but not in the way defensive play can.
I'll be skeptical about Grushnikov until the second he hits NHL ice and looks like he belongs, and if he tops out as a #6 type d-man who isn't doing much in transition or offensively, I'll consider the pick to have been a waste. At the end of the day it was another gamble on a guy who could barely get scouted in a weird year, so maybe not a big deal, but it's not a good look to put up such crappy numbers on the best team in the OHL, even if just by coincidence of getting some secondary assists.