The hydrodynamic efficiencies are a small part of the problem, there are many other things that can be done. You don't need to scrap a perfectly good hull, you can simply choose to run slower, burn higher-quality fuels, or strap on an oxidation catalyst (DOC) to cut CO, particulate filter (DPF) to reduce particulate emissions, or add selective catalytic reduction (SCR) to reduce NOx emissions.

But until regulation and the market catch up, old ships will choose to run too fast, to burn whatever's cheapest, and to vent their exhaust directly into the atmosphere.