On a lot of vintage plated horns, you see wear through to brass at the usual touch points (palm keys, side keys, pinky keys). Suppose you have a horn with perfect silver plate, how much regular playing would it take for the silver plating to wear through? And what is the best way to prevent this from happening?
The thing is, right in your opening query there exists two separate scenarios, in a way.
Yes, vintage plated horns do show wear to plating. But actually, if you think about it (and I dunno if folks are aware of this as much as I may be, since I obviously get a LOT more saxes in and out of my hands than most)...the vast majority of old silverplate horns show miraculously GOOD wear resistance, really. I mean, it's fair to say there's as much of a chance of acquiring a vintage silverplate horn which still looks really GOOD as there is acquiring one which has significant plating wear (I'd actually say MORE plated horns I have refurbed looked quite good as opposed to fair or worse).
I mention this only because....there seems to be significant difference in 'old' plating as compared to the plating quality on many newer horns. I have seen some pretty iffy plate quality on a fair number of new horns....I mean horn perhaps 10 years old already showing hues of the brass in areas which are NOT commonly touched.
So, in an instance of a contemporary silverplate job of questionable quality, you may well be in a no-win situation.
Now back to the crux of your question:
As you know, and everyone here as well, the plate wear is most typical at areas where the fingers are regularly touching the surface (spat keys, palm key touches, side key touches, octave key touch)...then next to areas where one commonly 'holds' or 'carries' the horn when it is NOT being played (front-ish of bell, back-ish of body tube, etc).
As noted by others, 'prevent' those areas from acquiring plate wear ? You'd have to not touch it, basically. That's the 'bad news'.
The 'good' news is (not so good really, but)...one can have bare brass areas which are developing....spot-plated (usually chemically spot-plated), although there is also an electro spot plating method which exists. This is somewhat ephemeral a solution....but I mention only because it IS available and it CAN 'bring back' worn areas to show as 'silvery' again....it just might not hold up as long, thus require re-treatments.
Therefore, the best solution is probably, as Turf illustrates, changing your state of mind: if it's an eventual inevitability, just embrace it as being part of owning a silverplate instrument.