Boeing has designed 3 new aircraft – 747SR, 747-300D and 747-400D – basically for 2 airlines (JAL and ANA).

No, those were adaptations of the existing 747 airframe at the time.

You make it sound like each was a whole new aicraft. They weren’t. Essentialy, those three models were just modifications to the MTOW and Range.

Adapting an existing airframe for a single purpose (or in this case specification for a small number of customers) is far far cheaper than starting a clean sheet design.

Boeing has a history of pumping out unique models of existing products for customers. The 707 for example had several different fuselage lengths, some designed specifically for a very small number of airlines.

Ditto for the 737