It’s not all fallback: it’s getting in the way of the actual fallback monospace/sans-serif/serif. The cargo-culting nature of this means that there tend to be one or two bad fonts in there, and sometimes some systems will have bad fonts aliased to otherwise good names (e.g. that’s why I say “don’t write Courier”, because although it’s fine on macOS and probably aliased to something reasonable on Linux platforms, Windows aliases Courier to Courier New, which is a terrible font).
I don’t want your idea of a suitable font, in general, because the default defaults (that is, the browser’s default value for sans-serif) are thoroughly good enough, and I changed my defaults, and by writing the likes of `…, Arial, Helvetica, sans-serif`, you didn’t change anything for most people, but prevented me from getting my chosen default sans-serif.
Long font stacks are fighting a fundamentally unwinnable battle with small potential benefits, high uncertainty, and definite costs (normally mild) to people that actually expressed preferences to their user agent.
It's all fallback, how can they be "not a good idea"? That's the beauty of these CSS font stacks: you can always put first the latest best practices.
> Most font stacks now are just cargo culting, and are mostly mildly harmful.
I disagree and if anything they're not complete enough (I'll write another comment).