There are limited circumstances in which you might cut him, but I think the thing with Sanu is:
(1) He's been a consistent 60/750/4 guy his entire 8 season career for multiple teams playing second-fiddle to a true number 1 receiver. Those are basically healthy Danny Amendola with the Patriots numbers. Through eight games with the Falcons this season, his pace was 70/700/2 playing despite the emergence of Ridley and Hooper. After the Ravens game, his pace was 80/750/4. Then he got hurt, missed a game, and played poorly when he returned (though at that point the whole offense was sputtering.) The guy's only 30 and isn't reliant on athleticism, so given that we have 7.5 seasons of data that he's a consistent 60/750/4 guy and less than half a season of data suggesting he's bad, we should expect that's really who he is.
(2) A $6.5m cap hit is peanuts for that level of production. Corey Davis has a $6.5m cap hit on his rookie contract for next season. Marquise Goodwin has a $6.5m cap hit and he has a fifth the career production of Sanu at the same age. Willie Snead has a $6m cap hit and couldn't even see the field in Baltimore. UFA contracts or extensions from last season include Adam Humphries ($9m/year, 37 receptions), Devin Funchess ($10m/year, 3 receptions), Sterling Shepherd ($10.25m/year, 54 receptions), Tyrell Williams ($11m/year, 43 receptions), Albert Wilson ($8m/year, 43 receptions), and Paul Richardson ($8m/year, 23 receptions). None of those guys had Sanu's resume and they all got more money than him. Sanu on the open market, even after the down half-season with the Patriots, would cost $10m in free agency.
So maybe you need the money for something else, but to replace Sanu, you're going to have to pay more than Sanu. The only fathomable better use for that money is if you need it to make a Beckham or Cooper or something work, and you're confident Harry (or Meyers, rookie, or cheap free agent like Dorsett) can pick up the slack as your WR3. A Patriots WR3 like Hogan or Amendola had been is usually looking at somewhere between 40 and 65 catches, 600-750 yards, and 3 or 4 TDs, though that's with Gronk and Edelman as your top two targets. (LaFell put up 75/950/7 in his best season as a the third target, so that's the high end of the spectrum but probably not going to happen again because that was a Brady MVP year.)