Dear Doctor, If a regimen like Combivir and Efavirenz can reduce a VL from 95,000 ( or more for some people )to below 20 in somewhere between three and six months, why, assuming you adhere to it perfectly, would there be cause to think the same regimen can't keep the VL below 20 permanently?

Your question is excellent - in fact many physicians believe that if the VL is reduced to less than 50 and the patient is perfectly adherent and no other factors interfere with drug absorption, then the same regimen should work indefinitely. Unfortunately, it is difficult to prove that viral replication has been shut down and is not just markedly impaired, that drug resitant forms of the virus are not present (from prior drug therapy or as a result of transmitted drug resistance) which might lead to a failure of the regimen, or that incomplete penetration of the drugs into certain difficult to reach sites (like the brain) would permit drug resistance to develop and ultimately drug failure to occur. All of these potential issues contribute to our uncertainty that complete suppression will persist indefinitely despite "perfect" adherence.