I agree that there’s a lot of pressure to get this movie right, but I’m not sure Spielberg would’ve been a good choice anyway, even if he agreed to this. I’d argue that the man’s past his prime.
One thing I know for sure is that messing up this movie would be a far bigger setback than messing up something like the Eternals. The Fantastic Four is the title that put Marvel as we know it today on the map, and they were once seen as the company’s definitive superhero team. Ideally, if this movie does well, I think the Fantastic Four should be the new flagship characters of the MCU.