Also, without a military the US would not be even 1/3rd as wealthy as it is today, given its military created the global order that secured the last 80 years of the global economic system, shipping lanes and USD dominance. You can argue over specific wars/missions being dumb, but to pretend the overall ROI on that dominance enabling 80 years of relatively peaceful global trade hasn’t been positive is to be intellectually dishonest.
The world is currently teetering on a global economic crisis over just ONE shipping lane not being fully open for a few weeks. Read more history and you’ll see this used to be the norm.
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".