I avoided commenting on the ROI associated with defense spending, deliberately.

Veterans get SS too, so no, costs associated with veterans wouldn't shift to SS. It is fair to suggest that the health care costs of uninjured, untraumatized veterans would just show up under Medicaid/Medicare. I don't know what percentage of veterans health care costs (not health care visits) fit in that category, versus "stuff that wouldn't be an issue if they hadn't been in the military".