Is taking antivirals every day considered a cure for herpes, or do the meds stop working after a few years?

Prevention not cure. Taking antiviral medication can decrease the risk of having a herpes outbreak but it does not cure or permanently eliminate the virus from your body. Unfortunately there is no cure for herpes. If a patient has frequent outbreaks then taking daily antivirals can be very helpful.
No. Daily suppression of herpes with antivirus decreases recurrent outbreaks of herpes. It also may decrease the transmission rate to an unaffected partner but it is not a cure for herpes. To date there are no reports of decreased effectiveness of daily use of antivirals for herpes either.