A lot of the Lego builds we did when my son was little was exactly this: A set would spark an idea and there'd be endless castles that started with piece of a castle set but went in totally different directions and incorporated all kinds of other stuff, for example.
I can't but help think that people who assume that the big sets take away that haven't touched Lego in decades.
My sons sets got built "to spec" once, got played with like that for a few hours, and then never looked the same again ever, even though we still have the manuals in a box somewhere.
I find most of my kids friends don't mix sets, each set stays in its own box/bag
That's borderline crazy, lego is meant to go into a big box/bag/container and be mixed so you use your own imagination to create your own stuff, like the lego gods intended. Ideally also passed down generations/between relatives, so you can visit your nieces and nephews in the future and recognize pieces you once received from some cousin or something.
Did the parents grow up with Lego? The "each set stays in its own bag" thing was what my wife wanted to do for the longest time, not having grown up with the beautiful chaos of a giant tub of pieces, and the oddly reassuring sound of swishing your hand through all of it.
That sounds really sad, and not Lego's fault...