What Happens to a BCC Left Unremoved?

You would be hard pressed to meet many physicians who would entertain or advocate leaving a diagnosed basal cell cancer alone. Best.Dr. Karamanoukian 

As others have already mentioned, BCCA is generally slow growing and indolent.  However, if given long enough, a basal cell carcinoma on the face will spread locally and can eventually affect nerves and even the brain.  Patients with chronic immunosuppression, such as with chronic lymphocytic leukemia (CLL), develop much more aggressive variants.  Given the cumulative solar exposure in SW Florida and an aging patient population, I have seen a number of patients with CLL and squamous/ basal cell carcinoma die from skin cancers.  If you have an SCCA/BCCA, it should be surgically removed within a reasonable time frame of diagnosis.