what has gone up is the number of vaccine exemptions. A child is considered to have an exemption if he or she doesn't have every vaccine in the series. As more Vaccines are added to the schedule, children will miss one or another. So the total number of exemptions is monotonically increasing as new vaccines are added to the schedule, but the rate of vaccination with MMR, which is all that's important for measles, is staying constant.
I've searched the scientific literature for papers presenting actual evidence that vaccine induced herd immunity exists without success. If you think there's any evidence for it, please post a link as a challenge to this proof. Please read your paper first as there are scientific papers that mention herd immunity as if it were a settled thing; however none of them present any actual evidence that it exists, nor any citation of another paper providing evidence that it exists.
There is good reason to believe it doesn't both because outbreaks occur in fully vaccinated populations and because the scientific literature indicates that most people can contract subclinical measles and pass it on a few years after their last booster so that most of the population is exposed to carrying measles and passing it on.
According to the scientific literature, most people have a vaccine titer low enough 2 to 7 years after their last booster that if exposed to measles they will very likely become at least subclinically infected with flulike symptoms but no rash, and they are contagious for measles. So measles can be passed around even in a fully vaccinated population.