The Problems with D&D 5e

Thats always a problem with CoC. Why would randos actually do the needful in said universe.

You can just play Delta Green which is basically CoC: Spec Ops edition, where the cannon fodder operatives are ordered to go said nightmare zone and do the needful. Gives people assault rifles to keep those martial types entertained.

Ah, so more the campaign planning instead of running the scenario in itself.

I honestly offload most of the work in regards of "Why do the PCs know each other and why are they here?" on the players. For the latest campaign that I started I told them "You are all from the same small city X and you know at least another person from the group. Also find me a reason why you would be in city Y and why you would participate in a guided museum tour?"

So the group came up with a doctor, a boxer, a coroner and a museum curator. Doctor & coroner know each other from uni days. Boxer was a regular patient of the doctor and I don't recall how the curator initially fit in (maybe just family friend of coroner?). Boxer had a fight in city Y, doc & cur were at a medical congress in Y and the museum curator's player asked if he could know the guide of the museum tour. No problem, that NPC wasn't very important anyways, so we upgraded him to being THAT museum's curator & being an old friend of the PC curator.

Until now no one has died, but since everyone is from city X (and the main part of the campaign is around X) it's relatively easy to replace PCs. The doctor's player as an example has established that his PC is sending reports about what they find to an old pen pal in England. That person could be a new PC - and is a good option for me to remind the players about things they forgot. Or someone from the local police station who is interested about what they mentioned earlier on in a police report etc etc

Another option are good source books. "Die Janus-Gesellschaft" (the Janus society) is a (German only) source book about a secret society. Either PCs can start as all members of such a society, or only one is or whatever and over time everyone can become a member. And since the society knows about the mythos existing (just like many cults) they can fund & help the investigators in larger campaigns. And other members are also an easy source for new cannon fodder (PCs).

The idea is basically: if you up the scope of the campaign from a small cult to a global conspiracy with many active players then you also need to increase player power. A single group of investigators has no chance there.