Kovalev doesn't have a bad chin at all, he's got your good chin, that has been hit by superior talented champion boxers like Ward and now Alvarez.
Kovalev has the same chin as Andre Ward, Floyd and other great world champions in boxing, it's just that the others like Ward and Floyd use defensive techniques to hid their chins and not get hit flush and when hurt badly unlike Kovalev they recover and use defense, to not get knocked out or stopped.
Now your other iron chin boxers like Evander and Tex Cobbs are a different story and your china chinned fighters are totally different from your Floyd"s and Ward"s and Kovalev"s.
Kovalev has the same chin as other all time great champions and if his chin was china and glass, he would've lasted that long as world champion and other top notch world class boxers would've already stopped him, like i said he's got your Andre Ward / Mayweather solid chins, but unlike him they use defensive techniques to not get hurt constantly and not get stopped or knocked out and Kovalev doesn't have these defensive techniques and doesn't bother to hire certain trainers to help him develop them in training.
Maybe his old trainer actually helped him defensively to avoid getting hurt badly and these newer trainers don't help .
Also conditioning plays a part in weak legs and not receiving from a power shot and his old conditioning and strength traner also said he's incredibly lazy and Kovalev said he actually drinks while in training camp, if true or not true, this might be the reason he's getting stopped in fights nowadays and has poor conditioning, but not drinking and still being lazy, isn't going to fix poor defensive techniques and getting hit flush, because everyone can get knocked out.
Just look at his abdominal core muscles during his last fight weight-in against Alvarez and you will see why he can't take body punches and doesn't put in the correct solid abdominal exercise he should be doing and then look at Floyd"s abdominal core in his weigh-ins and you will see the correct muscle core and constant correct strength training and conditioning it takes to get solid abdominal muscles.
Kovalev is lazy and doesn't put in the correct champions work in camp, to stay world champion and defensive techniques to not get knocked out, he thought his punching power would save him in every fight and knock everyone out and now it's come back to bit him in the ass.