My main issue with this approach is that it's relying on a lot from a guy who's coming off an achilles tear and has yet to play a down in the NFL. I would bet that this was the Patriots' plan after they drafted Wynn, with the idea that by this offseason they'd have a season of practice and playtime from Wynn to evaluate him and confirm that he's up to the task. But injuries would have got in the way of that, as they do.
If the Pats cut Cannon this offseason, they save $4.75M against the cap. That's definitely not chump change, and if you think you can re-sign Waddle for half that amount and plug him into Cannon's spot in the tackle rotation then maybe you'd be inclined to do that. Personally, as underwhelming as Cannon has been I think Waddle is much worse, and I don't want to see him back next year no matter which tackles are or aren't re-signed.
I guess my takeaway here is that I'd probably keep Cannon only because a) he's the only tackle on the roster who's played a down in the NFL and is signed beyond this offseason, and b) tackles are so expensive that you'd have to spend more than you'd save by cutting him to have any hope at signing a replacement who's a bona fide NFL starter. Or stated alternately: to replace Cannon with even a semi-proven commodity, within the budget of your savings from cutting him, you'd end up having to settle for someone who has a lot of the same question marks that Cannon has, minus the benefit of that guy having experience in your system. So for stability's sake, I'd keep Cannon one more year, see what we have in Wynn in 2019, and then probably cut Cannon after next season assuming the tackle situation isn't too dire at that time.