You've just answered your own questions... If you do not want to self-custody then many more than 100 million can have bitcoin. All 8.2 billion people can have it as long as you trust a 3rd party custodian.

And in the case that you don't trust a custodian you can keep it in your brain, or you can keep it in a vault, or you can burry it in your yard, or you can set up complex access with n of m keys being required to access it. You can do all kinds of fancy custody methods with bitcoin if you have self-custody options.