I'm pretty sure the original writers or whatever retained ownership of the franchise or has it worked out where he has to okay anything else BttF related. Assuming that gets passed on to whoever gets the rights after he dies, hopefully someone in his family, I could see the protecting the franchise because they know they can't do something that'll live up and they'll still be making bank on those residuals anyways.
"You need to lay off the peanut-butthurt and u-jelly sandwiches" - Neon Octopus