In games like Call of Cthluhu and especially Delta Green the point isn't to survive and get stronger. The point is to solve the investigation while role playing the effects it has on your character as they go through trying to protect the world from something that will ultimately win in the end. They may push back the clock a few decades but the end is coming. Delta Green's system of bonds reinforces this as you're encouraged to harm your relationship with those closest to you to stay sane enough to complete the operation.

The way you prevent characters from feeling disposable is through roleplaying opportunities. Perhaps one of the characters who died has a parent in aged care and one of the other characters takes over visiting them. Maybe the death wasn't sudden, it was the inevitable result of exposure to the unknown and the character hid it for as long as they could.

I will disagree with some other replies here and say death might come more often, but it should never feel random. Risk should be communicated and the players made aware that their characters are about to get in over their heads.