The current number of people who hold coins in self custody is the current number of people who are willing to overcome the friction involved with doing it. Ledger has the largest share, and despite all its problems, it has a great UX for new users. #Bitcoin only app builders should take note.
Discussion
I think part of the scale answer is Casa and Unchained and other "custody support" services. Maybe we need a non-profit (open source) Casa?
The issue with non-profit is there's no incentive to create a great product. Devs just build stuff however they want and say "if you don't like it fork it" not realising most users don't have the skills and knowledge to do so.
Very few bitcoin products end up being built with new users in mind.