There are a few things that hurt sets when they are refreshed. Newer sets are available at retail for far cheaper prices and usually are more morden looking (e.g. maybe more SNOT, newer elements, diff colors). People want sets for minifigs and special things included (e.g. maxi-sized minifigs). When a set is refreshed, most people stop looking at the brick elements included in the older sets and focus on the minifigs. If those minifigs are also refreshed, prices will further decrease because the older set is now looked as regular elements with just minifigs. sellers begin unloading the original set but unlike the past, they are competing against other sellers doing the dame thing,
Let's take the 2006 Batman 1 Batcave (7783) . This set is far larger than 6860. (It also contains the still exclusive Alfred minifig. Before 2012, 7783 was selling consistently for $350 used and $550-$600 new / sealed.). After 6860 as available for 6 months, 7783 prices fell too $225-$300 used while new sets hovered around $425 with rare instances of $500+. The sets bottomed out at $175 used and $425 new when Mr, Freeze was refreshed along with another Robin and Penguin. Prices have been slowly climbing again only due to 6860 being sold-out at times and the Alfred minifig. Additionally,, more 7783s were parted out than before so less are available for sale so supply vs. demand swing it's way again for a while but only after a deep depreciation. The Alfred minifig is still helping hold the value, This is just one example where a refreshed hurt the original's value big-time. If not for 6860, most used 7783 sets would be fetching at least $450 today even with the Robin and Penguin refreshes.
A more drastic example would be what the 2010 Slave 1 did to the original's value. The older Jabba's Palace can now be had for a song and dance. Even the superior designed 6210 Sail Barge is selling for far less than 1 year ago (used sets couldn't be had for under $200 in late 2012. I still can