"Long life" in the case of the classic 855 Cummins might mean three or four million miles or thirty thousand hours, and then finally being scrapped because of obsolescence rather than structural failure. In aircraft service an engine has a typical TBO of 2000 hours and a crank or set of cases might go three major overhauls.
The 5.7 Olds was actually at the end of its design cycle a reliable powerplant ....a fact few remember. Earlier ones had problems but catastrophic crank or block failure happened, if at all, at well over the 100,000 mile mark, when most gas counterparts had been replaced or the car junked.
The bottom line is that VW and Peugeot determined that the per unit weight and materials cost penalty was outweighed by cost savings in commonality of parts. The weight penalty is simply not colossal and it results in a gas engine that is more reliable and more rebuildable.