The 737 rudder hardover wasn't discovered until the mid-to-late 90s, a long, loooong time after the 737 entered into service. That does not quite speak to the benefit of a mature product (how mature is 'mature'?). Case in point: the 737 trim wheel and pitch trim assembly. The deficiencies of their design were put on full display in 2018, because of MCAS. Why do you think it is that (other) modern airliners do not have a stabilizer runaway non-normal checklist? There's a whole host of issues that are inherent to the 737 precisely because of how... mature it is. I think there's probably a crossover point, when a design ceases to be mature and becomes obsolete.
(Engines on newer models can be - and usually are - iterations of older designs. The A380's RR engines are built on the same platform as the Tristar's, introduced in 1972!)
(Engines on newer models can be - and usually are - iterations of older designs. The A380's RR engines are built on the same platform as the Tristar's, introduced in 1972!)