This is definitely true. I have a mental golfbag of games at this point, and I choose the game to suit the kind of campaign I want. I have MERP/Rolemaster for when I want to do gritty sim, I have WFRP for when I want to do something also sim but a bit lighter and more comical/tragic, I have Other Worlds (obligatory plug) for when I want to do something on the storygame axis, and I have AD&D 2e or red box basic for when I want to do actual D&D.Fair enough, but in a somewhat technical or analytic discussion like this one it is probably more helpful to say that D&D is widely hacked rather than that D&D is distinctively hackable.
Another factor here is that, once a person becomes familiar with a few non-D&D RPGs, they probably stop hacking D&D and use other ones that work better. Eg I gave up all my efforts trying to make AD&D more "simulationist" (systematised spell lists, a proper skill/proficiency system, etc) once I discovered Rolemaster, which does all that work for me!
And there are many other games on my shelf that I'd like to try one day, not to turn into something else but to experience them for what they already are (this includes my copy of Rifts!).