I could see that being true if it's an older actor (a legend in their 70s working on a movie guaranteed to be a hit) but he was in his 30's when that movie came out. Predicting that far in to the future would be stupid considering how much the entertainment industry has changed in the last 25 years (VHS, DVD, Blu-Ray, Digital, Streaming) and the fact that the production and distribution companies may not even be around anymore (I think in this case they are but that's a big prediction 25 years out). Also, if the internet hadn't come along the number of people rushing out to buy this on DVD would be much smaller. It's unlikely it would be in stock and by the time it was stocked people would have moved on.

So if you are factoring the actors death into your budget when creating the movie you're either a total idiot or willing to predict:

- An instant-seamless distribution system, aka the internet

- Your company will be around 25-50 years from now

- Robin Williams will become an immensely popular actor

- This movie will be so good lots of people will want to watch it in 25 years

Edit:

I can see this being more likely in the music industry as I think people are much more likely to go back and listen to an old album: they see an artist die, they think, "I used to love that band" and they buy the album. With movies, until the digital era arrived, I don't think people did that as much.