I think it'd be a non-starter for Portland. When comparing individual players, especially players that are expected to be at least the 4th or 5th best players on a contender, offense matters more than defense, and McCollum is superior offensively to Bradley. In addition, this is McCollum's first shot at starting, so he's likely to improve, while Bradley is likely close to his ceiling.
That doesn't mean Bradley isn't a great player to have. Above-average offensively without needing the ball in his hands, significantly above average defensively, still young, still on the books for cheap money, etc. But I think the Blazers would only part with McCollum in a package for a better player.