I'd guess that a given academic isn't going to face much of a career risk for signing a statement also signed by other very prestigious academics, just the opposite. There's no part of very divided US political spectrum that I can see denouncing AI naysayers, unlike the scientists who signed anti-nuclear statements in 1960s or even people warning about global warming now (indeed, I'd guess the statement doesn't mention climate change 'cause it's still a sore point).

Moreover, talking about existential risk involves the assumption the current tech is going to continue to affect more and more fields rather than peaking at some point - this assumption guarantees more funding along with funding for risk.

All that said, I don't necessarily think the scientists involved are insincere. Rather, I would expect they're worried and signed this vague statement because it was something that might get traction. While the companies indeed may be "genuine" in the sense they're vaguely [concerned - edit] and also self-serving - "here's a hard problem it's important to have us wise, smart people in charge of and profiting from"