Depending on the condition of the instrument, older telescopes can actually increase in value.
Telescopes are not products that "wear out" by gathering photons.
Many optical designs are not being made anymore for commercial reasons and are simply too expensive to make in the current environment.
Professional instruments can and do provide decades of excellent data with little or no change to their optics.
A customer of mine sadly kept his Astro-Physics AP155 under a tarp, like a lawnmower, outside on a verandah! The AR coatings were shot and focuser was riddled with corrosion. It was sold for peanuts.
Had he kept it in mint condition, it would have sold for more than double than what he paid for it new. Age is not a problem. Condition is indeed paramount and will reward the next owner with sublime data that a new "made to a price" rather than "made to a specification" instrument simply can't match.