Totally agree it should be available for cash pay.
But frankly, even if you are spending the money on medical expenses in the same year, you're also getting a lot of benefit out of spending pre-tax rather than post-tax money.
Post-ACA, high-deductible and HSA-compatible are not the same thing anymore. In my state, for example, there are no longer any HSA-compatible insurance plans but there are several high deductible ones.
If you require care for chronic medical issues, that often not even one treatment. So, the annual numbers might work out, but you're still left with a massive cash-flow issue at the beginning of every year (because there hasn't been time to fund the HSA, you have to float the deductible + co-pays/% up to your max OOP).
And that's assuming your income supports the max HSA contribution in the first place. Maybe after a few years, it balances out a bit, if you're lucky.