What will happen if I don't get treated for benign prostatic hypertrophy?

It depends. Many men have an enlarged prostate (BPH) and do not have bothersome symptoms. Some men need medical therapy for thier symptoms while others may need surgery for symptoms that do not repsond to medication or if they develop acute urinary retention and are unable to urinate. Your urologist will help determine the best course of action for you.
BPH . You may end up having complete obstruction of the urinary flow with reflux of urine into your kidneys causing serious damage to them, the bladder will be distended and may get perforated with severe pain or death, requiring at least a foley catheter or transurethral resection of the prostate.