I see what you are saying. Basically, they really had no resolution or conclusion for Mako or Bolin in that finale. And then they just kind of bounce leaving them behind.
Them leaving them behind isn't like THAT out of character. Er..given what their relationship is going into. But in terms of the overall story, it's kind of shitting all over Bolin and Mako. Pretty much, Bryke didn't give a damn about the side characters and it really showed in that finale.
Mako took the biggest L with that speech about loyalty. So much fierce loyalty. Korra then bouncing. Dude is a sucker.
In terms of logical plot though, Asami just lost her father. Korra is her "best friend"...and they are moving into a romantic relationship. So I can buy them leaving without the group. Still funny Bryke chose to make the other characters not matter in the end though.