Assumption: For the majority of people around the globe, the smartphone will be used as a wallet. There are people using feature phones to make BTC payments already, because they can't afford a smartphone. So probably, not all can afford to buy another hard wallet, and have the possibility to store this wallet securely.
Let me tell you the reality,
It’s Official: More People Have Access to a Mobile Phone than Toilets in the World which is shocking but backed by data so there is no point is assuming people are not privileged to have a smartphone.
Scenario: Imagine some people living in Asia 2004, having stored their BTC on a hard wallet, as every Bitcoiner suggest to do, maybe they also have some paper backup.
Within minutes, a Tsunami arrives, destroys your house, you are losing your friends and family, and if you are lucky, you find yourself and them a few days later still living.
This is why the community always suggest multiple backups at different locations which should be water and fire proof. I know not everyone is capable of doing that but we have alternatives like storing the seeds in meal sheets that can be worn as a jewelry, etc
Now, despite you lost your houses and your whole life changes from one day to the next, you also lost all your money. Everything, you put aside for later, maybe also for the future of your kids is gone. You have no chance to claim your private key or seed phrase. It's gone, unless you have shared the seed phrase with some trusted person in a different country / area.
How can technology help to fix this issue? How do you imagine this should be fixed in a BTC world?
This is rare phenomon and we can't really prepare ourselves for such incidents, no matter how good we are prepared we might not make it right at that time so we can have a backup that is stored in a different locations from where we live which depends on how rich we are and how much crypto we hold in our portfolio.
Let's say if it's just few thousand dollars then you could keep it in the same device that you use on everyday on good wallet that is self cutodial and open source. If it's in milions then you probably have ideas and where to keep them safe.
All that self-custodial thinking is useless in such a scenario. And that's just one out of many scenarios, when people suddenly have to leave their homes, and have no chance to take their smartphone or hard-wallet with them, because you may lose important minutes or seconds to save the life of someone you love.
I am definitely not a pessimistic person, but these are real problems BTC will face when it comes to mass adoption.
If you are still paranoid about a natural disaster is going to wipe all your wealth then it's okay to subscribe to those self custodial multi signature wallets which offers recovery by holding one or more keys on behalf of you.