Kotetsu is going to be so sad when he hears about Muichiro, but you know who I think will be even sadder? Kanamori.
He's undergone a recent, quiet journey surrounding Muichiro, who was not only a Pillar and therefore someone who inspires awe, but an enigmatic genius. To not only have been put in charge of his sword, but to be told to try to "understand" him, someone who gave no impression of wanting to be known or understood at all? That's double the amount of pressure.
Kanamori tried to understand Muichiro before Muichiro tried to understand himself. By that time, Tetsuido-a swordsmith whom it seems Kanamori greatly respected-had only left behind writings and specifications about Muichiro's swords. My sense is that Tetsuido only verbalized his worry for Muichiro to Muichiro that one time. Rather than writing, "please understand how hard this child pushes himself," perhaps Tetsuido only left hints to understand Muichiro through observing his sword.
And maybe through that, Kanamori looked deeper at the used swords and it dawned on him little by little. It's only natural if he started feeling concern for a youngster, however awe-inspiring of a swordsman he is.
And then comes the night Gyokko attacks. Kanamori fights to defend Haganezuka, is rescued by Muichiro, watches Muichiro be injured on his behalf, and then bears full witness to Muichiro's transformation mid-battle. What's more, he's taken aback by Muichiro thanking him for the sword.
Tetsuido probably never got thanked, and probably never saw Muichiro wield the swords he smithed. Given Kanamori's comments as he looks after Muichiro and Kotetsu and their budding friendship in the light novel, it seems Kanamori appreciated the deep concern Tetsuido had for Muichiro and was a bit emotional seeing how Muichiro was doing after such a transformation.
In a different timeline, Kanamori might had grown to be a fretting, doting father figure to Muichiro, concerned both for his physical and emotional wellbeing on top of being his trusted swordsmith who takes pride in furnishing Muichiro with Pillar-quality swords made precisely to Muichiro's liking. Maybe Muichiro would smilingly assure him not to worry so much, and tease him for being so concerned about someone so likely to die in battle any night. And making they could have had a swordsman-swordsmith relationship so open that when Muichiro comes to visit Kanamori could come flying and give him the fatherly hugs the boy must so sorely need all the time, even if Muichiro teasingly replies that he's fine. Maybe someday after Kanamori comes running through the village with tears of worry and relief in his eyes to give Muichiro a customary fatherly squeeze, Muichiro would point out how much taller he got than Kanamori.
Maybe Kanamori was happily starting to daydream about such a thing when he found out.