Very very good point!i think MS isn't only gainable by killing your best friend..
i mean Madara and his brother got MS by hard training everyday..
so why can't kakashi gain it that way aswell ?
If Kakashi dies in the next manga chapter or so, we'll never learn how he achieved MS.
As far as I know, it hasn't been revealed yet.
Also, he won't get a chance to teach Naruto about yin/yang chakra.
lolOne day naruto will return to real world and realize that his entire friends are wiped out.He will say some wanna-be cool stuff about revenge and forget it the next hour, then he will head to ramen shop.
And Pein is gonna get killed by sasuke as always :
"You ruined my village! I was the one to destroy it!"