Interesting. My financial planner has never suggested using my HSA as an auxilliary IRA. Maybe because he wouldn't earn any management fees from that....?

You’d have to also have a qualifying healthcare plan so you could have an HSA in the first place, and enough income to easily cover any medical bills without having to touch your HSA balance, after maxing out other tax-advantaged retirement options. Take away the first restriction and it remains a niche option, but does become realistic to aim for as extra tax-advantaged retirement savings on top of what’s already available.