Multisig ftw!

I don't think it is a matter of costs, but perhaps user education and awareness. Though things have vastly improved, most users still do not self-custody and choose to leave coins on centralized exchanges. That needs to change first before multisig can be more mainstream. So I'd rephrase your question as: how can we make self-custody of bitcoin more mainstream?

I think it will come, but will take time. This is a zero-to-one technology after all. Gold didn't emerge as the dominant form of money over night. It took thousands of years. I don't think Bitcoin adoption will take that long, but we are talking about perhaps decades. (Or who knows? maybe some major fiat currency will collapse tomorrow).

Demand will also naturally rise as the market cap of Bitcoin increases. You'd think twice about leaving a million dollars on a CEX or a singlesig wallet, vs. $50k.