Chrysler didn't really have a projected operating life. The best way to answer this is to say that Chrysler went from using a 5 digit odometer to a 6 digit odometer for a reason in the late 80s. Mainly because many of these engines will last over 200000 miles.
As a general rule, manufacturers give 10 years of support to a vehicle, as far as keeping the supply chain active for spare parts. That could be considered the design life. There's no specific goal set for most products, they are tested and the weakest points found can help project life.
People used to keep their cars for 7 years on average, but that's gone to over 10 years with this economy.
There's no more reason to junk a Neon or PT (which IS a non-interference motor piston-to-valve but can sometimes have valve-to-valve interference) for a broken timing belt any more than there is to junk a K car for a bad head gasket. In either case, the head can be repaired relatively inexpensively once it is off the car.
Honestly, the industry probably planned 10yrs/100k miles or so back then.
Somewhere I read a few years back that the design life of the powertrain was increased at Chrysler, but I don't recall from what to what.
That being said, it was easy to get more than 100k miles out of most cars of that era.
4 out of the 5 Spirits that have been in my family are still on the road. They originally covered '89-95 (one '89, a '91, a '93 and two identical '95's). The '89 was wrecked by one of my idiot cousins, and the '91 rear-ended another vehicle at a stoplight. They have seen all sorts of treatment, from owners who cared very little about their vehicle to those who truly babied them. All but one had over 100k
I'd guess the answer is more complicated. Probably it has to do with the manufacturer keeping up with industry quality levels relative to the level of the car in the model pecking order. Perhaps the warranty offered factors into it. The manufacturer has to present an image of competitive quality and warranty, or they get into trouble. It comes to mind that Chrysler has indeed run into that problem.
That said, it seems to me the Spirit had parts that wore out or needed repair too quickly (the v6 valve guides), others that gave average service, and others that were even unnecessarily durable, like the stainless steel exhaust system.
How a car is maintained can be more important than the build quality. People keep every sort of car running.
As I contemplate buying a Prius and observe their amazing repair record, the Spirit's record seems pretty bad. But ok for a car of its age and original price bracket.