Let him test the market, if he gets that, let him walk. Otherwise, max I’d give this guy is like in the $12 million range. He doesn’t do a lot of things great to get that money. He’s very undersized and not a great rebounder or defender. He’s become a good 3 pt shooter but we’ve seen past history that it’s not smart to just overpay for good 3pt shooters. You can find specialist players for cheaper than that.
Again though, the question isn't whether there are guys who do similar things around the league making in the low teens or less, it's whether
Boston can find a player to replace Grant if he walks. You don't open up any ability to sign someone else for 10-12 million a year, because you don't have cap space and you're too far over the tax to use the non-tax mid-level or work a sign-and-trade. You're stuck with only the taxpayer mid-level to replace Grant, which you had to use
with Grant too - it prevents you from plugging a hole or taking a flier elsewhere.
Theoretically you could spend some (increasingly scarce for Boston) second-rounders to convince the other team to do a sign-and-trade to create a TPE, and then later try to trade for someone to replace Grant, but you're burning (at least) a first and two seconds just to save Wyc some money.
Any way you cut it you're taking tools off the table over $2-5 million a year, you're not executing any big-brain strategy to improve the team. It's penny-wise, pound-foolish for a team chasing a ring.