I had been hacking the open source quake 3 engine while in the community of Urban Terror, a closed source game on top of it. I believe a problem that studios would face with what you are suggesting is that there is still a lot you can control at the low level, ranging from cheats to game behavior.
PS. Of course one could claim you can never be certain of those (hacking disassembled code is always an option) but in any way you look at it, if you do have the resources to have your own engine you retain more control.
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I had been hacking the open source quake 3 engine while in the community of Urban Terror, a closed source game on top of it. I believe a problem that studios would face with what you are suggesting is that there is still a lot you can control at the low level, ranging from cheats to game behavior.
PS. Of course one could claim you can never be certain of those (hacking disassembled code is always an option) but in any way you look at it, if you do have the resources to have your own engine you retain more control.