> Ground loops have never been a problem for headphones.
No, but they're a problem for consumer hi-fi systems, which use the same 1/8-inch RCA connectors.
> And SPDIF over optical (presumably you mean ADAT) is considered vintage now.
Yes, but it still exists, and you still have to consider its existence in the design of a modern component that could be hooked up as part of an audio reproduction chain, especially a component of a consumer hi-fi system.
My point was that the signalling standard which modern devices connected via a headphone jack have to obey if they want to be able to talk to "anything else that has a headphone jack", is complex, in the same way that e.g. USB-C is complex. Everything "needs" active circuitry just in case the other side isn't playing/expecting plain analog audio over the jack. ("Needs" in quotes because you wouldn't expect a $10 pair of headphones to work if you plug them into your CD player's SPDIF-out port, but you generally would expect the same of a pair of Bluetooth headphones that have their own internal DAC.)
> Bluetooth is simply not a professional audio technology. It's not enough of a standard, and not reliable enough even when it's implemented correctly.
Sorry, didn't mean to imply that Bluetooth was good, or that it was suitable for professional usage.
What I meant to assert, specifically, was that when comparing the signalling standard of the Bluetooth audio profile, to the de-facto signalling standard required on e.g. a smartphone's audio connector, the Bluetooth standard requires around the same number of active components at the [antenna] connector terminal, as the headphone-jack de-facto standard requires at its physical connector terminal, to achieve the same level of user-expected interoperability. (And something like Audio-over-Ethernet is even more complex in terms of the number of active components required!)
So, while Bluetooth might not be good, I wouldn't describe the thing being mourned here (the de-facto signalling standard backing the 1/8 inch headphone jack) as being very good either. It's also complex, it's also "not enough of a standard" (because it is extended with things like SPDIF and TRRRS connectors), and it's also frail—and comes with a host of problems of its own, like signal degradation over short distances and wire frailty, when implemented at the wire gauges consumers [rather than professionals] gravitate toward.