I love to kitbash, taking parts from multiple kits to make one model. I got heavily into 40K during what may have been the golden age for that sort of thing, 2000+. As GW moved to more and more full plastic kits, either by design or by happy accident they delivered up a goldmine for kitbashers, by including extra parts and making kits largely interchangeable, so that you could mix and match various marine kits, chaos marines, fantasy, etc. to a large degree with little effort. They also released various upgrade kits like the wonderful Space Wolf sprue, metal bits for subsets like Iron Hands, Iron Warriors, Thousand Sons. And they did bits orders, so you if you wanted a particular metal piece or a particular sprue you could often order it directly instead of having to buy a full kit. Plus Forgeworld of course.
While GW's own contributions were wonderful, that era also saw a rise in other companies making compatible kits and conversion bits, online bit sellers and the explosion of the glory of ebay, where you could pick up someone's whole cast off bits box. I was in heaven.
A number of my armies are a result of this bonanza of bits, using loyal Space Marine parts from various chapters, chaos marines, fantasy parts and pieces from other companies like Kromlech.
Many kits died to provide the bits for these guys. Chaos marines, berzerkers, space wolves, two generations of fantasy chaos warriors, dark elves, bretonnian men at arms, word bearer metal pads, chaos terminator combi-weapon, ork shoota with hand flamer bit added, 2nd ed metal models and some old backpacks, probably more.
A selection of Space Sharks using marine, CSM, iron warriors, sanguinary guard, steam knights, berzerkers, dark eldar (2 generations), forgeworld, space wolves, ogres, orks, RTB01 bits.
But it's not all marines, though they probably have the best selection to play with from GW's own range. Above are some of my Lost and the Damned-style Scythiak, mixing Catachans and various chaos kits in the top pick. Lower pic some fantasy ogres with some ork weapons and CSM bits, notably the defiler masks.
Overall I think the future of kitbashing is still bright. While GW themselves seem to be going further towards extra bling and iconography on parts making them less universal, and some kits that have parts that fit together in ways making interchangeability harder, they continue to put out lots of parts, and the dual-use kits help make spare pieces even more available. More and more companies are putting out their own kits and bits and expand their ranges, and ebay is full of people chopping up and reselling kits.
So save those bits! Or give/sell them to someone else who will use them. Spread the bits and the love.