Even if the filming was finished by the time Neville fell from grace, his tarnished reputation if the public found out might put a damper on the film's popularity.
"There is no escape, John!"
So long as his extracurricular goose stepping hadn't been made public, I'm sure the studio would've rushed to cash in on his death; "Neville Sinclair's final film!". But in the years that followed I'm sure word would've eventually leaked, or rumors whispered, and it would likely have fallen out of favor into the public domain.
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A journey into the realm of the obscure: http://saturdayshowcase.blogspot.com/
Neville's death was already being covered up (Alan Arkin reads the newspaper account of Neville's tour car being crushed by the falling flaming debris) so of course Hollywood would take the ghoul route and release it and market it as Neville's farewell. As to how Neville's goosestepping would leak...dunno.
Jamie Lee Curtis survived Halloween, the Fog, Prom Night and a Terror Train & now she can't poop!