I honestly feel that if a speaker is really good, that you won't see it on the market that often as most will keep it. That said, this is also audio where so many change their components like some change underwear, lol.
If a speaker has been out for many many years, then you will obviously see it being sold as there are just sooo many of them and anything can eventually get long in the tooth due to newer tech adn materials etc...
I also see a ton of some companies speakers always available as they change their designation every couple of years. Wilson comes to mind. Takes nothing away from their speakers, but that they change models often.