Cap hit isn't the issue. Actual money is what counts. Cannon's cap hit is currently projected to be $9,622,916. But his dead money is projected to be $5,133,334, and that number's not coming back to the team. So cutting him this year, while saving some money ($4,489,582), would leave the team with having to fill the starting RT role for $4,489,582 or less, in order to achieve any savings.
Here's the list of scheduled FA tackles. This is the group from which you've got to find a starting level RT who'll play for that $4.5m slot, if you're not going to turn the job over to either a pick from this year's draft or one of last year's wounded rookies.
2020 NFL Free Agents: Offensive Tackles
And there's no Scar around this year, so the idea of taking a lesser player and coaching him up is a lot more questionable.