You make some good points that must be taken into consideration. None of us should be surprised that Diggins is leaving.
I might not precisely recall the circumstances, but as a casual observance, I believe Diggins was the first recruit in this year's freshman class. I do recall his noted reason for deciding to commit to UConn was about how Hurley had challenged him to come here and take his game to the next level and help UConn get back to elite status. Diggins then went on to actively recruit Hawkins and maybe Samson Johnson as well. Diggins may have thought that having a good relationship with Hurley would help bring him along as a player and DH would nurture him to the next level. Exposed to the cold reality that his performance in practice (most likely, rightfully so) simply did not get him any significant game time, without a beneficial relationship with his coach, Diggins concluded that it was time to move on, get a fresh start and try it somewhere else. I seem to sense that Diggins play, while not stand-out during very limited action in the early out-of-conference games, further atrophied as the season progressed (like walking on egg shells). If DH was fully intent on developing this high level recruit, he could have shown his commitment by putting Diggins in for 2 to 3 minutes during the first half of games, early on in the season and later as well. Given Gaffney's limited performance this season, all the more reason to invest in Diggins. Now an upperclassman, Gaffney has been given playing time during his years at UConn to grow and improve. Gaffney became the starting point guard as a freshman when Alterique faltered. But when RJ Cole became eligible to play the next year, Gaff seemed to hit a roadblock in his progression. And when he was a bright spot on the team in the disappointing loss to Maryland in last year's NCAA tourney, there was hope that Gaff would elevate his game this year. But that did not happen in any significant way even though Gaff was given the benefit of the doubt for much of this season.
Diggins never got a real chance to play any significant minutes this year the way Gaffney did in his freshman year.