I heard that at some point GW tried to stop the selling of armies on eBay because it is really undermining their business. When they found out they couldn't, they made sure to release huge 150$ sets that you would need to make the army competitive and also Nerf the old units down and make then less desirable.
Sold off my tau that I got in 2009, but hanging onto my marines that I got 2 years ago because I spent a lot of time customizing them and painting them.