If I understand correctly, Grant can be extended after this season or he becomes a RFA in 2023 with a $6.2M QO. I am not so sure that the big offers will be there for him as an RFA. Look at Markkanen. He struggled to get his deal at $16.8M average and he would seem to have more market value.
Grant has been a useful, versatile player for us. He dramatically improved his 3pt shooting that has helped is value significantly. But his ceiling is still likely a bench player or maybe fringe starter. He is an undersized player whose best season is 8 pts, 4 rebs. Markkanen is a near 7-footer who as a starter, flirted with 20 pts/10 rebs. I am not sure what all this means in terms of actual contract value. Players always get more than I think they will get, always. But maybe 4 years/$50M? That is already more than RWill, he got 4/$48M. So maybe actually 4/$45M?
Maybe the best thing for him is to play out his final year and become a RFA. If he keeps improving, who knows. I just don't see a lot of teams opening up either check books for him.
You can't look at how a single player like Markkanen (whom I continue to think you overrate, as I would take Grant over him, but that's mostly beside the point), did in free agency without considering the context of the free agent markets at the same time. Firstly, the salary cap will be roughly 15% higher when Grant reaches free agency (barring the extension I think the Celtics should pursue). Secondly, and more importantly, very few teams had real cap space last year. Only 8 teams had $16.8 million in cap space entering free agency last year; Markkanen had few teams who might be willing to come in for his price range without having to work out a trade with Chicago, and only four teams (Knicks, Mavs, Spurs, and Thunder) had $30 million or more in cap space as of August 1st (meaning they could sign Markkanen and other players). The Knicks had the most space at $52.6 million.
In the summer of 2023, there are currently 16 teams that are currently slated to have $55 million or more in space. In other words, half the league is on pace to have more cap space than the league-leading Knicks did last summer. That's going to come down a bit as there will be some free agent signings, extensions, and draft picks between now and then, but it won't come down nearly as much as some years due to the pretty limited free agency class this summer. Probably about half the league will have the cap space to target Williams, and he fits a role that most every team is looking for. The market for his services is going to be substantially larger than Markkanen's, and with more interested teams, his contract will likely rise relative to Markkanen's. Meanwhile, despite a lot of teams having cap space, the unrestricted free agent list is a bit underwhelming, so I expect some teams will make large offers, perhaps frontloaded ones, to RFAs. This is especially likely to be true given the cap spike I mentioned above. If a team has a ton of space in 2023, spending above market for a player that you nonetheless like, and putting him on a declining deal will make him a bargain contract once the cap spike hits mid-deal.
I do think a deal will get done this summer, because Stevens has shown a willingness to sign players to extensions, but I also think it will come in higher than a contingent here thinks is reasonable.