3) Barnes reportedly wants a bigger role. There's no indication Green ever wanted to be "the man." That Barnes hasn't been able to take a leading role on arguably the best team in NBA history shouldn't be seen as an automatic strike against him.
If he had the ability or the inclination to be the man, to take a leading role on a non-Warriors team, or to shine just as bright as the lead, then he would've shown it every so often, even on the Warriors. The occasional game when he enters beast mode and plays like the man for a night. Has that ever happened? In 305 regular games, 244 starts, he's scored 30 once, exactly 30, in the last game of the season back in 2014, with Curry sitting out...and he was outscored by Jordan Crawford, who led the team with 41, off the bench. He's only had maybe 2-3 other games in his entire career that can be described generously as alpha games. 1 out of every 100 games or so. Wait, there's the playoffs, too...40 games, 33 starts, out of which he has scored 20 or more points 4 times, all in his rookie season.
Check his game logs. I would be interested in seeing another player in recent NBA history who's had that many minutes in that many starts, who's had so few big games, and who then went on to be a big-time scorer. I don't think one will be found.
Harrison Barnes would be a risk, sure. Obviously KD should be the first target as a SF. But Barnes is a risk worth taking if we can't get KD.
We don't have to make a big free agency splash this summer. Aside from Durant, the pool is weak, including Barnes. He'd prevent us from signing someone next summer, when the pool will be deep with stars, including maybe Durant again. Better to just keep both max slots open.
I get what you're saying regarding Barnes not taking over much, if ever. But even looking at history, his situation is completely unique. On most every team in the NBA he'd be either first or second in three-point shooting amongst players with 200+ attempts. On the Warriors, he's fourth. And the Warriors know who shoots the best -- Curry. And then the shots go to Thompson, who's second. Then Green, who's third. Then Barnes. One of Barnes' offensive strengths is simply redundant on that team.
For perspective, Curry and Thompson combined have taken more 3s themselves than 6 other NBA teams have. And the two of them have made more 3s than 14 other teams. The Warriors as a team shoot 41.5% from 3, which would be 10th in the league as an individual player. In the Warriors offense, Harrison Barnes taking a 3-pointer is a bad shot. He shoots 38.3% from 3, better than every other team's average. Barnes taking over a game would be an example of Harrison Barnes playing selfishly and outside of the offense in Golden State. Those other young players who dominate occasionally have never been in Barnes situation. Even Harden in OKC was coming off the bench at times, being allowed to lead the second unit. Barnes is a starter, but it means he has to share the floor with players who will (rightfully) dominate the scoring chances.
It does make him incredibly difficult to evaluate, because there are no comparisons. So you look at his age, his physical measurements, and the fact that he can shoot, and hypothesize that this is someone who can succeed in a larger role.
But I reject that Barnes will foreclose the Celtics from other opportunities the following season. Just as most every team will have cap room this summer, so will every team have cap room the following summer. Barnes would make about 85% of the 0-6 years max in 2017. Barring injury, he would be tradeable if a better free agent wanted to come to the C's. Furthermore, the last two CBA negotiations have included an amnesty provision. Even if Barnes were for some reason not tradeable, it is probable the C's could erase his contract that way if truly needed.
It is time to stop putting most of our eggs into the following year's free agency basket. This team is good enough to make long-term additions who have the ability to grow within our team and offer a talent upgrade over our current roster. Barnes is one of those players. Just because he's not a top 10 player in the NBA doesn't mean we should hold our noses. It is unlikely we'll ever get such a player in free agency, because such players change teams rarely. Furthermore, we have two offseasons of cap flexibility left. Once the summer of 2018 hits, we'll have to deal with IT, Bradley, and Smart in free agency at the same time. Beginning that year, free agency will be about maintaining the status quo and not making the team better.