It's strange how you say Thomas is a "good" 3pt shooter and Granger is an "ok" one even though Granger shot better from downtown last year. I'm going to assume that you meant we would have TWO good 3pt shooters in the starting lineup.
I wouldn't call Robinson a poor outside shooter. I would say he is an inconsistent outside shooter. He has solid shooting form, and with his work ethic, he should be just as good of outside shooter as Thompson was. It might not be tomorrow or a month from now, but you can't discard him like there is no hope for his jumpshot. This is a young team. Many of our players still have weaknesses. With time, they will improve on those weaknesses and improve the team in the long run.
This is how I break down the shooting for our current starting lineup:
Thomas - Good 3pt shooter, good midrange shooter
Evans - Poor 3pt shooter, poor midrange shooter
Johnson - Poor 3pt shooter, poor midrange shooter
Thompson - Poor 3pt shooter, good midrange shooter
Cousins - Poor 3pt shooter, good midrange shooter
This is how I break down the shooting for our starting lineup after the trade:
Thomas - Good 3pt shooter, good midrange shooter
Evans - Poor 3pt shooter, poor midrange shooter
Granger - Good 3pt shooter, good midrange shooter
Hayes - Poor 3pt shooter, poor midrange shooter
Cousins - Poor 3pt shooter, good midrange shooter
As you can see, our starting lineup will have more shooting than before, and most importantly more 3pt shooting. On the second team, you have Brooks and Fredette who are good 3pt shooters, Garca and Salmons who are okay 3pt shooters, K. Martin who is a good midrange shooter, Robinson who is an okay midrange shooter, and Johnson and Outlaw who are poor shooters.