Apple was saved because it abandoned all its pre-1997 software and adopted NEXTSTEP, which Jobs created.
I'm also reminded about the state of HP once the founders retired. My understanding is that John Young and Lewis Platt preserved the HP Way. However, Platt's successor, Carly Fiorina, moved the company far away from the HP Way, and HP hasn't been the same since.
This is a sad time for NVIDIA. I hope NVIDIA is blessed to get a Platt and not a Fiorina.
If someone else that wasn't directly trained by Jobs for years was hired, i'd argue that the culture would start to falter because one thing slides somewhere, then another, then another an soon enough 'A players' don't want to sacrifice their core years at a company that isn't instilling the values they want to work under. Cook himself almost went down that path with his budding career at Compaq. His family thought he was nuts to leave the growing company for Apple of all places. They didn't see the values that Cook saw in Jobs.
I don't think that Nvidias board will have any problem though finding an A tier CEO to run the company.
They probably will not get an S-tier visionary, but mature companies with deeply established roots don't necessarily need that like nascent companies do.
Maybe the veneer isn't there, but it's very hard to make the case that Apple has in any way failed following the loss of him. Cook was pretty mediocre, and it shows that when you have a strong ship sailing, you just need a "not total idiot" to keep it going.