It’s certainly an open question. I don’t believe 8 billion people will be able to hold bitcoin in self custody, but 8 billion people don’t all hold any scarce asset right now.
The rate of development is so fast too! I think current state were not there for masses having self custody if prices and fees continue on current trajectory, but stuff like this that mutiny wallet and others are developing certainly continues to help. https://primal.net/e/note13y66pezucap8tjjf80q2mzmc387u6e3jujzk97736lmck972m8ns00s3j7