Yes, it's becoming harder and harder to find a 10K. I'd love it if the popularity of 10K races picked up but I'm not sure I see it happening.
The marathon will, obviously, always be around and I don't see its strength declining. The growth rate has to decline at some point but I don't think we'll see declining participation. The half is a nice still long compromise for those who can't or don't want to do the full distance. The 10K/5K are both relatively short events compared to the *marathon distances but, in my opinion, the 5K wins out as the easier/shorter/more accessible distance. The 10K seems to be not long enough to get the respect but not short enough to be considered accessible enough. The 5 mile/8K seems to be getting some love but not a ton. The other distances (4 mile, 15K, 10 mile, etc.) just don't seem to be getting any love and, already rare, are doing well if they just maintain.