Motor oil is so much better than it was 20-30 years ago and that’s a large part of the reason that 300k is the new 100k.
I think electronic fuel injection and modern high-power ignition systems have more to do with it than the oil. With the carburetor's near-total demise, piston rings and cylinder walls no longer get washed-down by unburned fuel to the same degree as in the past.
Oil stays cleaner, bearings last longer, engines need very infrequent tune-ups. Apart from the oil and filter, maintenance is down to an occasional can of fuel injector cleaner in the tank.
The "threshold" is now parts availability. It helps a lot to own something that has some degree of "enthusiast" following; that tends to mean there is broader aftermarket support for parts long after the manufacturer has written off the vehicle as obsolete.