Thus, like an old clunker of a car, those surplus oil tankers are cheap because they are nearing the end of their useful lives. Whatever the Navy spends to acquire, rehab, and modify the tankers must then suffer rapid depreciation. How long will these navalized, missile-spewing Tankers of Death last in service? Ten or fifteen years, with lots of babying and maintenance expense. Then they must be written off. Budget analysts will be highly skeptical.
Second, against modern naval weapons, the damage absorbing capacity of oil tankers is much less than it may seem. Modern missiles and bombs can easily penetrate the unarmored sides of a tanker and explode in the vitals of the ship. A single torpedo will commonly be fatal by exploding under the hull and breaking it in half.
Due to the innovations of naval weapon designers in fashioning more lethal warheads and more accurate missiles, the example of the Atlantic Conveyor containership during the Falklands War is no longer relevant. Yes, after a hit by an Argentine Exocet missile, the ship lingered for days, but that would not happen today. One and done is far more likely against any unarmored civilian vessel, even a large tanker.
The basic problem is that without armor, watertight compartments and bulkheads, damage control systems, and defensive anti-missile systems, large civilian ships are easy targets these days.
First, I doubt very seriously that a single torpedo could break the back of a VLCC. It's just too big and built too stoutly.
Most newer tankers are double hulled, for instance. They are built tough to avoid oil spills.
As I noted, I wasn't talking about just strapping missile systems to the main deck: the ships (including propulsion) would have to be thoroughly overhauled, redundant power and command centers added, comms, and, of course, CWS. Outer bulkheads would have to be reinforced, perhaps some armor added.
Such ships would have one overwhelming advantage compared to regular Navy ships when it comes to weapons and armor: size. Heck, you could put in more bulkheads and just fill the outer compartments with water and you'd still have an enormous interior space.
Stuffed with long range land attack and ship-to-ship missiles, as long as it was within range it would retain major offensive capability even if seriously damaged.