While in the UK a few years ago I met with one of the bigger British militaria dealers. To quote him " A true collector does not sell anything, only acquires". In some cases this is true but most "collectors" do add and subtract over the years from my experience.
About 6 years ago while speaking with a well known Canadian militaria expert he discussed his thoughts on his collection as he grew older (plus 60 years of age). He had no kids or relatives to give anything to once he passed away. Museums would generally pack things away, never to see the light of day. His solution was to thin out the collection over time, allowing other less mature collectors to acquire items they might never see, while receiving compensation that allowed other ventures to take place.
I took his advice and with the help of an advanced and honest collector here in Ottawa I was able to thin my militaria down, injecting a sizable collection of uniforms, headdress, cap badges, flashes, webbing and assorted odds and ends back into the collecting fraternity. I've been able to maintain a manageable collection and also use the extra funds to support other interests including the higher end items I wanted.
I guess it depends on ones wishes once they are gone, but seeing another 50 collectors enjoying the fruits of my searching is OK with me. I can't open a museum so thinning it down (while I'm able to guide the destiny) worked for me.
As with anything we collect be it old cars, militaria, stamps etc nobody really appreciates it as much as we the collectors do, so I guess ultimately as long as we can help decide on the next caretaker it all works out.