This argument only works if there isn't fading immunity (the 1/6 rate....the range is probably somewhere between 1/3 and 1/8 and we don't really know because of the time element...1/6 may be a blended rate based on where population immunity is at any one time) from vaccines and the 1/6 is enough of a slide down the R to prevent outbreaks (which given what's happening in Iceland, Singapore, Norway, Finland and Ireland is a little bit dubious right now). If vaccine protection against infection slides down to near zero after some time, unless you are going to mandate boosters constantly (and if people are off the train now, imagine what they'll do with constant boosters) this doesn't hold water. You'd have to compare someone's ability to transmit unvaccinated to post vaxx 3 months to post vaxx 6 months to post vaxx a year. If that year number approaches the unvaccinated transmission numbers, your argument collapses.
You also have to draw the distinction between unvaccinated and unvaccinated but prior infection as those rates are going to be different.