Self custody is still possible on layer 2. In fact it will probably be the default for the majority of people, or even custodial solutions from trusted third parties that are kept honest by layer 1. They don't have to be there own bank to be banked. I think things are looking very positive for the global south if they can take advantage of Bitcoin and get out from under the thumb of western colonialism.
Discussion
The best, established, proven self-custodial avenue is literally becoming prematurely expensive to enter.
I don't think it will matter for most people.
If self-custody cost-of-entry is still tied to some sort of on-chain activity, then how is cost of transacting on-chain not relevant?
You're right, but whether it's self custody or custodial, Nigerians and other impoverished countries will benefit massively from using Bitcoin. There will come a time when they are paid fairly for the value of their labor and resources and then the cost of self custody won't be such a hurdle.