HSAs functionally act like a traditional IRA from a tax and investment perspective (although there are not many good places to hold an HSA account currently from an expense perspective) with the added benefit that it could be used for health expenses tax-free. This would actually put an HSA above a traditional IRA as a priority for your investment money. (Roth IRAs depend on tax bracket.)
Many functional early retirement strategies require loading up pre-tax vehicles, shifting to a Roth over time and sustaining on Roth contributions that are in the clear. Health expenses are probably the most common reason for unexpected expenses. The HSA provides flexibility for you to pull from it without penalty in retirement (or anytime, really) without going through any additional hoops, thus providing more flexibility to access your money.
In your hypothetical example, you'll run into health expenses on your way to FIRE as well. In this case, you have a $1000 expense you will either pay from your HSA or your bank account. If you pay out of your HSA, you will have an extra $1000 in your taxable account that would likely make its way into a Vanguard fund eventually. That money is available to spend however you like, but you'll be taxed on the dividends and any capital gains you incur over time. The HSA money grows completely tax free and remains 100% eligible to use for medical expenses.
Another good note on HSAs is how they can serve as emergency funds for health services no matter what stage of life you are in. If you find yourself temporarily unemployed with a few health bills piling up, this asset remains available to you.
Not much can get into the HSA, so it's not likely to be your dominant asset. If you work 10 years, you'll add about $35K that probably grows into $60K or so if invested. I could easily see myself generating this much in medical expenses, but if I don't, it's eligible as IRA money when I am 60 anyhow. As long as I have significant tax assets outside of retirement accounts, I don't really plan to use it. I might change my tune in retirement and tap it occasionally. However, I can understand the point of view of someone who has to make tradeoffs; I am capable of maxing all available tax shelters (as a W-2 employee anyway) and thus do not face the issue of comparing investment vehicles for best use of my money.
I certainly wish we could just have a generic pretax vehicle to use however we wanted. All this IRS complexity is a pain in the ass, but sadly worth the time (to me, anyway).
