The thing is, if everyone is vaccinated then adults/teens don't get it as commonly either. It is a short term worry. It's not as though chicken pox is lurking in the air waiting to pounce on anybody unprotected, you get it from contact with an infected person. Children are currently the most common group to get chicken pox, so if few children get it (because they are all vaccinated) they won't pass it on to adults or teenagers either.
I suppose there is a problem with the first generation of vaccinated children that there will be a time where some slightly older ones won't be vaccinated, but won't have caught it either. We are in Germany and there is a two phase programme happening here. DS1 is 10 and his peers were vaccinated (though he was not as we lived in the UK at the time) but children ~5 years older were not, and anyone 1 year older than him would have only had a single dose, whereas a second dose is now recommended. So any child who has never had chicken pox is invited to be vaccinated between the ages of 9-13 to cover that second vulnerable period. DS2 is 6 months old and will get the combined MMR plus chicken pox vaccine when he is old enough.
As a result of the change, chicken pox is rarer (though not eliminated) here and DS1 hasn't had it.