I think the first thing to look at is how much material is left on the base and edges. The layer of p-tex which forms the base is fairly thin, from 0.7 to 1 mm. Each time when skis are ran through a wet belt sander and stonegrinder at a ski shop, a noticeable fraction of this thickness is removed. Some ski shops are more careful than others, but it does happen that a ski shop would sand the base until all scratches are gone to make a customer happy (who does not even suspect that this procedure has shortened the useful life of the skis). Likewise, sharpening of the edges from the sides, especially with a regular file instead of a diamond file, reduced the useful thickness of the edges.
Snow also gradually grinds down the bases, but my take on it is that it would take hundreds and hundreds of days on the most abrasive snow to remove a significant base layer thickness.
So, in my opinion, the first thing to look is width of the edges from the direction of the base and from the sides. Ideally, the edges should be rather symmetrical in thickness from the base and from the side, and a couple of millimeters thick in both directions. You can go to a ski shop and check how the edges look like on new skis. Each time the base is ran through the wet belt sander, thickness of the edges decreases together with the base. Keep in mind, though, that p-tex is likely thinner than the edges, so it is not about how much edge is left, but about how much edge is left compared to what you see on new skis.
I never run skis through the sander (do base flattening manually if needed) and I have skis which show only minor tear and wear even after 5-7 years. But I can imagine that one can run them to the end of their life with the help of some ski shop equipment within a year.
The second parameter to look at are big dents and nicks on the cases and their repair status. While small nicks are easy to fill, the big ones are tricky since p-tex repair strings and candles do not exactly match the strength of the original material.
Finally, the next weak component in skis are bindings. These sometimes fail. I had a case when 6 year old bindings on my skis failed release test and had to be replaced. Older skis with integrated bindings are at most risk in terms of repair-bility - if a binding component fails, it will likely not be possible to replace or fix it, and no new bindings may be available to put on the rails built into the skis.
I think most skis get replaced because something newer and greater came to the market or because they no longer look "sexy" rather than because they reached the end of their life.