It may not be that simple. The virus is clearly circulating and may be present asymptomatically in vaccinated individuals who can transmit it.
http://www.ncbi.nlm.nih.gov/pubmed/9700638
Those who are fully vaccinated should benefit from a subclinical infection due to a boost to their immune system.
Although viral transmission between protected individuals has never been directly demonstrated, the data describe a population in which protected but infectious persons could potentially be of epidemiological importance.
If one doesn't see transmitted cases or clinical disease, then effective protection has been achieved. If lack of natural boosting was a significant issue with measles, we would have already seen lots of cases in people born before 1957.
None of those abstracts describes clinical infection or transmission. They simply describe antibody spikes after exposure.
There are several issues here. One, it's not unusual for antibodies to spike when an immune person is exposed to an organism. That's how the immune system prevents you from getting sick.
Two, with some organisms waning immunity is a problem and with others it is not. There's a lot of work going on now to determine if natural boosting (being exposed to cases) plays a role in preventing shingles after chicken pox infection. And it isn't clear why, (waning immunity, vaccine failure or new strains), people who've had 2 doses of MMR are getting mumps in some outbreaks.
Three, in some cases, infection occurs but no disease. Hepatitis B vaccine does not require boosters at this point even when antibodies become undetectable if they were detectable in the past. Vaccinated people occasionally turn up with core antibodies proving infection occurred but after 30+ years these vaccinated people are not being found with any active infection.
Four, there are a lot of infections where the antibodies simply are not protective. That's why HIV and hep C have no vaccine yet.
Five, vaccines and past infection don't always protect against new strains. There actually is a rare strain of hep b that the vaccine does not protect against. A unique strain of measles could turn up though I'm not aware one has been found.
And finally, as I mentioned, an individual's immune competency can always be the problem rather than not having antibodies against a pathogen.
If failure to naturally boost measles immunity is going to be a significant problem, we will begin to see more cases in people over 55 as vaccine levels fall below the threshold for herd immunity in more populations. I'm not aware we've seen it yet. But the more actual measles cases that occur, then those more rare infections begin to show up. The bottom line, we still don't need people over 55 to run out and get MMR boosters.