Disappearing characters/Relationships

Just finished binge-watching the entire series and I thought it was fantastic. I was surprised because I am not a huge football fan and assumed the series to be just another very unrealistic teenage-bop One Tree Hill type of show (sorry, OTH fans).

But what I found myself irked by is the consistent disappearing of characters who I felt were gradually shaping up to be important characters in the show... until they were dropped entirely/mysteriously fall off the face of the earth, and/or had underwhelming storylines and character development. It's irritating because you grow to like these characters, and then they just leave. I almost feel ghosted.

Examples:

Waverly: Smash's girlfriend who was dealing with a bipolar disorder. They made a great couple, and she truly allowed Smash to show off his softer side. To my knowledge, there was no explanation as to what happened to her. Please correct me if I'm wrong. Smash began seeing another girl, who is (IMO) much less interesting and then that was pretty much it.

Santiago: Love his story and wish they continued. When was he able to move out of Buddy Garrity's? What happened to him? I think it would have been interesting to see more of the friendship between him and Lyla.

Hastings: I forgot his first name. He was the "hippie" football player who originally had no interest in playing football until Coach Taylor recruited him. Could the series have gone on without him? What was the point of his character? I truly thought I was going to see more development within his character. And his first appearances on the show made me believe something was going to develop between him and Jess. This was a tease.

Lyla: Lyla is certainly not my favorite character in the series, but I'll admit, I really loved seeing Lyla and Tim Riggins together as a couple. I'm a bit disappointed that the writers made it seem like Tim gave up on her as easy as he did even if he did claim to have loved her, and even if she did have to go away to school.

Landry and Tyra: Granted, they were a very unlikely odd couple. But for all that they had been through together, I wish the writers had included at least one last scene in which Landry and Tyra tie up any loose ends, amicably if anything at all. I loved their relationship/unusual connection and I just don't think the writers did them justice in the end making us believe Tyra really left Landry in the dust once she left for college.

Julie Taylor: As for Julie Taylor's character, I thought she had extremely poor character development for the amount of airtime her character receives. I think of all the characters, she evolved the least- coming out as not much different than when she came in. Her entire appeal (to Matt) isn't very believable.