I think my comparison was too absurd/convoluted, because you're still making the same fundamental categorical mistake of asking "Why can't [X] be treated as [Y]?", as if "food" and "drugs" are just synonyms, and not separate categories with different rules and preconditions. But in any case, in a universe where "steak and eggs" were classified as a drug, and have also been commonly sold over-the-counter (i.e. at your local diner, without needing a doctor prescription), then nothing would prevent your doctor from prescribing "steak and eggs" for Alzheimers. And in our current universe, nothing stops your doctor from prescribing Viagra or Tylenol to treat Alzheimer's, even though none of those drugs have been approved by the FDA for that specific use. Doctors have a lot of freedom to use their judgment in prescribing off-label uses. So it's not a question of "Why can't doctors prescribe steak and eggs/Viagra/Tylenol" for my Alzheimer's — it's "Why won't...?", which I think the answer is mostly self-evident.

So why can't Aduhelm be treated like an OTC herbal supplement? Well for starters, because Biogen didn't research and develop it with the intention of marketing it as a herbal supplement. So I think the answers to your question are very much related to the answer to "Why doesn't Biogen release Aduhelm as a herbal supplement?"

Obviously we can dodge the question of "Why can't [X] be treated as [Y]?" by saying "X is not Y", as you've done. If you ignore the point behind the comparison, and we say that "X is X", then there is no argument. Biogen still has time to do the trial to show effectiveness and is going through a process that is governed by the set of rules set forth for drugs (not herbal supplements).

Sorry, I didn't say you can't consider Aduhelm to be an herbal supplement, just that every premise and factor in the current discussion would be rendered irrelevant.

So OK, let's agree that Aduhelm is now an herbal supplement. The FDA no longer has the authority to ban it from Walgreens. What's your point? The FDA's decision is about Aduhelm's proven effectiveness as a drug, and it has no bearing on whether Biogen wants to sell it as OTC herbal supplement. Do you believe the FDA is what's preventing Biogen from doing that right now?

The OP was saying how by allowing the accelerated approval, the FDA failed to protect people from a drug that hasn't yet been shown to be effective. My point was that it's not like the FDA protects people from snake oil anyway. Doctors can already get their patients to spend tons of money on unproven treatments. As long as the treatment isn't lying about what the evidence has shown, adults and their doctors are able to make decisions based on available information, and generally society is ok with that. That sentiment conflicts with OP's sentiment. That is why the comparison is relevant to the discussion.

And in your hypothetical example, should insurance companies be required to cover this treatment? In other words, should other insurees be required to pay for it?

It depends on whether or not the insurance company determines it's of value enough to their customers to include it in their insurance package. Insurance companies don't insure all prescription drugs.

That would be considered off-label prescription.

Which is commonly done, and legal, right?

Yes, doctors can prescribe off-label uses for basically any already approved drug. I'm not sure how it would apply to Aduhelm, which AFAICT has no indicated uses other than Alzheimer's.

But let's assume there is no legal barrier to the doctor prescribing Aduhelm, and somehow you've found a doctor who is going to against the status quo based on inconclusive studies. Which insurers are covering it (certainly not Medicaid or Medicare)? Which pharmacy is stocking it? And where are they getting their supply? Biogen? You think Biogen's lawyers and executives are going to accept high legal exposure (e.g. consumer lawsuits) for an infinitesimally small market?

FDA approval provides a legal and market framework for drugs to be supplied, marketed, and paid for. If your friend who runs Biogen has a spare box of Aduhelm and gifts it to you, it wouldn't be the FDAs problem.