Maclean's Music also retired undefeated lol. So did Mastery. So if we are going by 'undefeated' what you're saying is Justify is only two races better than Mastery.
The lower the number of starts, the less meaningful/relevant the word 'undefeated' is. It looks good on an advertisement or stallion page, I suppose, especially if you spin it like 'Only Triple Crown winner to retire undefeated' I don't think being undefeated actually adds to his value. Especially when one could also easily point out the dubious honour of being the only Triple Crown winner to retire with less than 10 starts...
Plus, to put this in perspective there are horses who aren't undefeated but have win streaks in excess (sometimes well in excess) of 6 races. In the last few years alone you've got American Pharoah, Songbird, Beholder, Winx, Tepin... California Chrome had TWO six-race win streaks. Gun Runner had a 5 race win streak so not quite 6 but they all came in G1s, so he's got more G1s than our latest TC winner. Caravaggio was undefeated in his first 6 starts. Heck even Dortmund was undefeated in his first 6 starts.
The most valuable thing about Justify is that he is a Classic-winning, Dirt-running son of the late super sire Scat Daddy, and who won the Triple Crown. That he won another G1 besides the TC also adds to that. That he is undefeated doesn't add anything in this case. It'd be far more meaningful if he'd ran out the rest of the year and remained undefeated. Or if he had come into the TC with more prior starts.