We've been caching for only a few months - found a dozen caches and planted three. There are 360 caches within 30 miles of where we live - so even if we find one or two every weekend, it'll be years before we have to drive more than 30 minutes to get to a new one. Since the area we can cover grows as the square of the distance, we will go on for a decade if we're prepared to drive an hour to the next cache.
So - at least if you live in a reasonably densely cached area, it should be a long time before you run out of new caches.
If people create and archive 10% of the caches every year, we'll never run out!
So I doubt that's the reason people give up.
I suspect it may be a matter of the difficulty.
We are already finding that difficulty 1 and 2 caches are just too easy. I hate it when you drive half an hour to get somewhere and find the cache within the first 5 minutes. This seems to happen a lot.
The answer (I think) is to come up with much trickier multi-caches, things with complicated clues that require more searching up-front. Sure we need simple caches for the beginners to find - but the ratio is all wrong.
To that end, we just planted a five-stage multi-cache with no lat-long for the first cache (just a cryptic clue), a puzzle to solve and a bunch of tasks you are supposed to complete at each cache.