If you watched the debates, he addressed that. He voted on and supported Roberts on his “known” paper trail. That’s why he prefers all future candidates have a long paper trail.
Maybe not the answer your looking for but that how he explained it. Long paper trails didn’t work out to well for Bork though.
I've long debated with myself on whether it is better to place old judges with a conservative record on the court knowing they will stand firm. Or seek young conservatives that will be there for a long time, hoping that they will remain conservative.