Is the possible death of a solo founder one of the fears in the mind of investors while funding a startup? Do they really consider this as one of the risks? If yes, then how can they be sure that if there are multiple founders, all of them will, let’s say, not die in a car crash? And how can they themselves be sure that they will live on to enjoy the returns from a successful startup?
While I am sure that may be the case, its more about preventing burnout. With a team they know that in theory they will motivate each other to keep running. But most investors actually invest in the people and their will to succeed when times get tough and shit hits the fan.
Dying is just an illustration of the point that they have all the eggs in one basket in the case of one founder. Many other things can happen, change of mind, burnout etc.
Honestly one of the main reason is having more than one founder gives the investor more leverage. If there's one founder and he owns 70% of the company, the founder has control. If there's two founders and each owns 35%, then the investor can play one founder over the next one or at least nobody has full control.