I've said this before but I'll repeat it. He's getting paid fairly if you compare him to other mediocre/above average players who got deals in that 8-10 range. He's the definition of middle class. It's more of a philosophy issue on how you choose to break down the salaries on your roster for each type of player, whether it be superstar, 2nd/3rd tier stars, and role players.
My problem with the deal is that I feel like that money could be better spent on either:
A: a player with potential to be a star or just below star status making 10-12/year, like Green a few years back when I think most Celtics fans thought optimistically he could be a little better than he has been. Green was athletic, a former 5th overall pick, had good measurements, and could score. He could be the 3rd option on a good team. Bradley should be no better than 4th or 5th option, and some might even argue on a really good team he's a bench guy.
B: two players in that 5-6 million range that are also above average and possibly even better than Bradley.
I get that Ainge just wants to keep his assets but the Bradley deal might hurt flexibility going forward. Like with Green the first three years of his deal when he was inconsistent, I'm not convinced his deal is easily moveable.
Furthermore, I was skeptical about how much Bradley would improve. With any deal for a young player the hope is that the player will improve or at least stay the same. His numbers this year are actually worse so far.
Since he doesn't have great athleticism, measurables, court vision, or a great handle, there are limitations on his ceiling. He could basically just improve his shot and get smarter with experience, but he's always going to be undersized and have low playmaking ability. In other words, a nice role player.