The guidelines are not to treat any lesion less than 70% (exception would be if there are symptoms such as chest pain), if there is treatment for symptoms, then
medication. If
medication does not effectively treat the lesion, then a stent implant. Sometimes stents are not a solution because the lesion is too long, difficult to assess, etc. then a bypass.
COURAGE study has concluded there is no difference for longivity with any of the 3 therapies (google for more information if interested).