Newtype_A said:
Got to remember that starships--and space ships in general--don't have as much wear and tear on them as, say, cars or boats or even modern naval ships. Their hulls won't rust, there isn't any friction to degrade their hull plating, and generally experience very predictable, uniform hull stresses so that metal fatigue can be easily rectified. Other than that, a few engine parts--plasma injectors, warp coils, warp cores, etc--would need to be replaced periodically from use, but starships would be designed so this is easy to do.
Consider that the Mir space station lasted better than twenty years with only ad-hoc maintenance and lacking anything close to a major overhaul. Other space craft like Voyager and Pioneer are still functioning to this day, and many satellites have ten to twenty year lifespans. And then there was Hathaway, which remained adrift for decades after being abandoned; I think most of the things that were wrong with Hathaway when Riker's crew went aboard were probably the result of whatever it is that caused the crew to abandon it; a couple weeks in space dock and the ship would probably be ready for duty anyway.
I would think that most of the things that can go wrong on a starship are easily repaired, but the ship would have to stop off every five to ten years at a starbase for big things like warp cores, computer cores, etc. Every twenty years, a MAJOR overhaul, mostly a refit of internal spaces and equipment (and a new bridge and equipment) but also, sometimes, the exterior of the ship is redesigned too. If they're good at keeping up with the maintenance, a starship should be able to last a century or more.
I disagree. Not because any of your points are wrong, but rather because you ignore the idea that there are many OTHER issues in space.
For instance, non-metallic materials... certainly all polymers, and many ceramics... degrade VERY rapidly in exposure to a hard vacuum, particularly when combined with exposure to the unfiltered radiation that's seen outside of a planetary atmosphere. Metallic materials tend to be somewhat more viable in this sort of situation.
A starship, as portrayed in Trek, is subjected to accelerative forces far in excess of anything we've ever seen... to the point where they have to rely in inertial dampening forcefields to keep the crew from being turned into red smears on the walls. We've repeatedly had dramatized sequences when a ship's hull was overstressed to the point where it was "nearly failing."
Plus, plus, exposure to phaser and torpedo fire (even if filtered through deflectors and shields, SOME of it always gets through doesn't it?). All varieties of radiation. Flights through all sorts of nebulae. Dipping into the atmospheres of planets. You name it.
So, yes, it's true that you eliminate some of the atmosphere-and-gravity-based issues in space. But Mir was in a net-zero-acceleration situation throughout its whole operational life, excepting when it was launched and when it fell back to Earth. Short low-acceleration "correction" maneuvers pretty much were the limit of the causes of mechanical strain applied to Mir.
Truth is, we are still infants in the "space survivability" field of study. I've deal a tiny bit with some of that... some materials compatibility work. Other people know a lot more about it, but I know enough to know that it's a very difficult issue to deal with. Space is an extremely hostile environment... every bit as hostile as the oceanic one. Not just for humans, but for hardware as well.
Ultimately, the "real answer" here is that ships last as long as the writers want them to survive, we all know that, in Trek. But if we're going to try to think about this logically, let's consider all the issues... including those above.
