> A brand new design would probably introduce more serious flaws than an interation of a mature design

As evidenced by the ever-crashing 787s and A350s.

The 787 was rushed through as well with the same modern Boeing safety culture and we were merely lucky that none of the flaws discovered in the 787 were bad enough to cause fatal accidents before they were rectified. Read up on its battery issues if you want an idea of what could have gone wrong.

I rather had the exploding engines of the A380 in mind, the flaming batteries of the B787, but we can also talk earlier B737 (rudder, rivets on early models), DC-10 (cargo door), C-5 (door lock), A340 (freezing pitot tubes) etc.

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!)