There is no question that conventional oil can protect an engine properly with regard to wear rate. That's never been an issue. However, today's engines are very expensive, and some engines have known issues. Various Toyota engines have trouble with oil drain holes on the piston oil ring clogging. The heat, combined with the small holes cause them to coke up, leading to oil consumption at relatively low miles. "Expensive" synthetic oils prevent this problem. Same with the famous Toyota "sludge monster" engines. Again, expensive synthetics prevent and/or minimize the sludge buildup and completely eliminate the problem. AND BMW, Audi, Ford, GM etc engines all have "known" oil related issues, that are effectively eliminated by quality oil. Consider the case of the Ford cam phasers. They are known to fail, and create an annoying engine rattle. Higher viscosity synthetic oils prevent the problem, completely. Same goes for GM's balancer chain wear issues, which require more frequent changes and slightly higher viscosity.
Not to mention turbocharged engines. Not only do they work hard, but the turbo's do still "coke up". Water cooling does not eliminate the extreme heat on the exhaust side of the turbine. Most people are unaware that modern, water cooled turbos still coke up. They are simply designed to be tolerant of it. Right up until sudden failure.
So if you drive any of the modern cars with known issues, you are well served by choosing an expensive oil, of sufficient viscosity, and changing it at the severe service interval.