But for me, I think Kori had the perfect type of personality to coach Mirai. Kori also knows what it's like for people to doubt her/underestimate her and has had her share of naysayers over the years.
Also it's worth noting that Jason also struggled with URs, though not as a chronic as Mirai's. I remember at a JGP in Japan back in 2010 he like like 7 < between two segments of the competition. Part of the struggle was due to a growth spurt (he had grown 4 inches between nationals in 2010 and that JGP). He also had a slight flutz during his junior career.
But you never hear about that now....Kori did a great job managing Jason's growth spurt and helping him get over any major issues that could have become chronic issues in seniors. Tackling Mirai's UR problem would be a greater challenge, but I think if anyone can hack (in the good sense) a strategy to conquer it, it's Kori.
And Mirai also brings something to the table -- fairly good speed and ice coverage, two things that I think Jason needs to up his game more.