Ignoring, for the moment, the eye-rolling "rock-star developer" legend: What is their contingency? At this point they likely have many such developers, so unless they're dumb enough to pack them all on a bus that gets driven into the Grand Canyon when the bus driver has a heart attack on a sight-seeing tour it's unlikely they'll all disappear at once. But when you start that way, especially in an "entrepreneur" environment, there's no time or budget for documentation, training, etc. and a lot of knowledge tends to stay inside that "rock stars" head; if they're gone the knowledge is, as well.
Of course, if these are "true" rock stars they have all of their ducks in a row and miles of documentation so that any other "rock star", or even just above-average software engineer, could immediately pick up where the last star left off. But I doubt this CEO can actually quantify any "rock star"-ness, so I would not be surprised if a sudden brain drain leaves them in a lurch due to their reliance on "rock stars". In the long run I expect it would be better to hire five competent engineers that can take over for each other rather than one "rock star" at five times the price. Maybe you'll get less "Aha!" moments within the company, but I don't think it's worth-while trade-off.
(This is forefront in my mind as I have experience with the potential issue: I am not "rock star" level by any means (perhaps above average at absolute best and with a pinch of salt), but I am the sole multi-stack developer at the small company I work for. If I go, so goes maintenance for the hundreds of small processes that keep the company upright. Many would likely continue without issue, but a few wouldn't and none of the other employees (mainly accountants) could even begin to work on them. It's something that causes me a lot of anxiety; I'm trying to improve documentation, train employees how to research data issues on their own, I've been hounding the owners for at least one other co-worker to not only ease my normal burden but also be able to take over if I fail to come in.)