I could pontificate without an end in sight about how BJJ and judo are the same thing. The sports are different, but the martial arts are one and the same. However, as more schools of each teach solely to a specific set of sport rules, huge holes are left in the grappling style of each. For this reason, I’d say the modern incarnations of each really need each other.
In judo we see a loss of the ground game. As soon as you hit the ground people turtle. Of course, there are those individuals who have phenomenal ground games. But typically we’d point to Travis Stevens and Flavio Canto – both of whom also have BJJ black belts. I’ve had discussions with other local and regional referees, all of them black belts, who didn’t understand that the omo plata (sankaku garami) was the same structure as a kimura (gyaku ude garami) and therefor attacks the same joint(s). They felt the omo plata was a shoulder lock but the kimura was an elbow lock. The reality is both are allowed if the arm is kept at a more or less 90 degree angle since this can be used to dislocate the elbow (as the hand goes behind the back in either it should be stopped by a referee as a shoulder lock though).
In BJJ we see a loss of certain fundamentals. Not only in standing, but in grips and ukemi. If you go on r/bjj it’s easy to find discussions where purple belts were surprised to find out that the predominant fingers in a gi grip should be the pinky and ring finger. To me, that would be as bad as a judo ikkyu/shodan not knowing how to do an armbar. Proper grips and falls aren’t concepts only important for standing fighting, they’re safety issues because you can still be thrown and will likely still take gi grips on the ground – grips which people may strip and an improper grip not only makes stripping them easier, it puts your fingers at a higher risk of injury. Of course, there’s also the notion that judoka are somehow magic at applying pressure in pins. It’s not magic, it’s a concept BJJ should be teaching and most advanced BJJ practitioners pick up, it’s just that judo explicitly teaches it.
You don’t need to stop and get an advanced rank in whichever isn’t your primary sport, but I will say that cross-training should be strongly encouraged. And unless you’re training in an old-school version of either, you’re likely creating holes that should be patched, not just for completeness, but for safety.