Thing is, now they've shown a willingness to make this change, so from here on out people will be worried about them trying it again at some point.

The only way they have a hope of putting this genie back in the bottle is to provide a loud, strong, clear mea culpa, stating that they were categorically wrong to propose dropping SMS support, plus a strong promise that they will continue supporting SMS for the life of the product. Maybe something along the lines of, "if we ever propose dropping SMS integration again, you can consider that a warrant-canary type of alert".