Why does the DC-8-70 have the same MTOW as the DC-8-60, despite its more powerful engines?
But maximum takeoff weight is determined by engine power


not by engine power alone. The airframe also has to be strong enough for the intended takeoff weight. There is a structural strength margin, but this margin is mandated by the certification authority so you can't use it. The design won't have much more than the required margin because that would make the aircraft heavier than necessary and less competitive. 

At a minimum, you'd have to get the aircraft certified at the new MTOW, which requires expensive testing. Worst case, you have to modify the design of parts like the wing box/spars, landing gear, and fuselage structure. 

By 1988 the writing was on the wall for narrowbody 4-engined aircraft, with ETOPS-180 coming into force. 


  It's the maximum weight at which the aircraft is physically capable of attaining liftoff speed on an infinitely-long frictionless runway at zero density altitude with one [for twinjets] or two engines [for more-than-twinjets] inoperative;


No. That would make the definition of MTOW useless in the real world. Compare to the FAA definition: 


  The Federal Aviation Administration has defined Maximum Takeoff Weight (MTOW). The MTOW of an aircraft is the maximum weight at which the pilot of the aircraft is allowed to attempt to take off, due to structural or other limits. 


The MTOW you see in manufacturer specifications is a figure established in the certification process. This will be below the physical maximum weight the aircraft can handle, to make sure there's a margin of safety.

In many circumstances (airfield runway length, altitude, temperature) 


  an aircraft may not be permitted to take off at its MTOW... The maximum weight at which a takeoff may be attempted, taking into account the above factors, is called the maximum permissible takeoff weight, maximum allowed takeoff weight or regulated takeoff weight.