I can't speak for anyone who thinks Manu should be getting $5-7 million per year, but I've generally thought Manu's contract would be in the range of $7-10 million per year (the lower end of that scale being when he looked like a s of his former self earlier in the year, and/or a lesser amount accepted in order to receive a longer 3-4 year contract).
This rationale had little to do with Manu's actual play on the court. Manu is an absolutely spectacular player who gives a ridiculous amount of effort; if you went solely on that, he probably deserves to be a top 20 NBA player in terms of salary. However, the main concerns with Manu are age, injuries, and style of play.
-Most NBA swingmen begin to decline after their early 30s, so his long-term value has to be a concern, especially with additional miles put on by increased professional ball at an early age and long playoff runs.
-Manu has been fairly injury-prone recently, and some of those injuries have come at very inopportune times (i.e.- late in the season). A player's value goes down if he doesn't play 75+ games, especially if some of those games missed or impacted are in the playoffs.
-Manu's style of play is reckless and only has one gear. In most cases, that's an asset; however with an aging, injury-prone player, that can be a detriment and shorten the length of his career.
As I said, I'm thrilled about having Manu back next year and a couple years thereafter, even if the price tag turns out to be $40 million for 3 years. But teams have to take all things on and off the court into consideration, and for the reasons above the price tag might be a tad "high."