The cap Pats currently have now are mainly due to the COVID optouts and other veterans getting cut. Before that there was less than a million in cap. In fact, BB had to restructure Rex Burkhead's contract just to be able to sign Cam for vet minimum. Did you somehow miss all of that?
As far as your other points, while Dugger has been solid so far, he lacks the coverage skills that Chung has. Also, while Onwenu has been a pleasant surprise, he hasn't played RT this year, so Cannon would still be useful. Not to mention with injuries at OL, he would be huge needed help for depth. Once again, you're way off.