You're right that adults will lose immunity (it's one of the biggest problems with WC) and that to achieve herd immunity there would have to be mass vaccination campaigns across all ages. Although I agree that the number of cases are quite small by historical standards (and actually have gone right back down to pre-outbreak levels in most areas). I think the CDC considered this early on in the latest outbreak and recommended cocooning as well as pregnant women getting the jab. I think the UK didn't consider this cost effective however, and there was also a shortage of vaccine - I found a year or so ago that you couldn't even get it privately as the NHS had sequestered all of it.
The jab in pregnancy isn't about herd immunity though, it's supposed to generate a short term response that's designed to carry the baby over till their own jabs at 8 weeks.
