Personally, I have five scopes, ranging from 500 MHz to some 50 GHz. When busy doing projects, older scopes are mostly gathering dust and are not worth enough to be bothered sold off. Also, one might need an extra for some unknown future reason. However, I just bought a new 13 GHz Agilent Infiniuum so I will sell my older 6 GHz 54855A Infiniuum since too valuable as dust collector.
I suppose one collects things one doesn't need when one has more money than time (to sell).