I look down. "Ummmm, I did..."
"Yeah, some homeless guy was really angry about that. He spent a long time beating on it."
(They sounded kind of traumatized about having to witness this!)
In summary, those "Fortified" lights were really good and I wish it was still possible to buy them.
I can't swear by its crackhead resistance, but I have one of these, https://www.amazon.com/gp/product/B01MG7677G/, and it's possible it might work for you. Reasonably inexpensive and available both on and off Amazon; it's quite bright, but has tamer settings too.
The included mounting methods are unsuitable for your purpose, but the light's body is metal and has a standard 1/4-20 screw thread, bolts for which are available at absolutely any hardware store. In lieu of an internal battery, it has a short coiled USB-A cord and a small pack you can bring inside to recharge. For my bike (a detail photo here for reference: https://www.flickr.com/photos/28626887@N05/41633010941/in/album-72157695957257125/) I used a small strip of aluminum bent over itself to make a bracket, with the light bolted to one side and the bracket secured through the handlebar clamp.
The aluminum strip slots neatly between two ridges on the light to prevent it from rotating, and the fastener is not readily accessible unless the crackhead in question has some time and is in the habit of carrying a 7/16 wrench with them. More exotic screw heads are also available if that's a problem, as well as Loctite and/or epoxy.
That cable-and-battery situation looks very crackhead-susceptible.
What made these Fortified lights great:
Yeah, the Fortified setup clearly had a bunch of points to recommend it. I don't claim this one to be feature for feature with the Fortified lights, just that it continues to exist and could work, and is reasonably priced.
In this case the battery goes with you, which is a downside if the battery can't go with you (or you don't want to take it), but is an upside re: passersby turning the light on, and it can also be run with a USB power bank.
Vagrants could certainly damage the dangling cable though, you are correct. My hope would be that with half the product visibly missing and the remaining half clearly nonfunctional without it, it'd be less of a target. But here in Detroit people steal Dodge Chargers, not bicycles, much less bike lights. So I can't speak firsthand to how it would fare on the mean streets of SF.
Let us know if you find one you like!
If I wanted to detach and carry something with me every time I locked up the bike, I could use literally any light in the world!
I know how you feel about no longer being able to procure that perfect item. It's almost to the point where I can't find anything quality anymore.
Acceptable use of a time machine:
Send a message to my past self at the time of purchase "Hey - buy like 10 of these. You're going to love it and get killer utility from it, but once it wears out, you won't find one nearly as good, because things just keep getting made shittier in the future."
I got that message and bought a bunch, and it was my continued custom that kept them being made -- then I noticed that it wasn't a message through time but across a quantum grating.
It's a long shot and it's obvious so I'm sure you've already thought of it, but:
Does Pig Monkey, from [previously, a year ago], have a light like the one that got attacked, that you can buy/beg/borrow/steal?
(I used to say things like "Survivorship bias!!!" when people would say that things aren't made like they used to be, and be able to back it up with facts. It's often not that way anymore: Things like bicycle accessories are shit, and they're getting shittier. I therefore have no more-modern replacement to suggest.)
For my preferred use of my bike light (different priorities/threat model, etc.) there's now about one type of front light "works ok, not perfect, but definitively not shitty" left on the market. The rear light... I might as well just go for the cheapest option and buy a spare right away so "I don't feel too bad about the money when it inevitably breaks". O temopra, o mores.
The kludges I’ve used to protect accessories against theft would be too awkward for handlebar lights. It looks like Fortified’s design hit a great balance between security and convenience. It’s unfortunate they aren’t made anymore.
You can drop the “useless battery” requirement for a replacement. I doubt the thieves could even make that distinction. They’re just going to steal a shiny object and hope they can fence it. I’ve had lights stolen that aren’t very useful without their matching and easily removed bracket yet the thief didn’t bother taking an additional second to strip the bracket off of the bike too.
I have a front and rear pair of this style (along with four batteries with questionable life left) that I'm no longer using. I got tired of the batteries slipping off the contacts mid-ride, especially with the rear one. When Fortified was still around they'd ship a little rubber spacer to stick on the batteries that would theoretically make things better, but it didn't work well. Anyhow, I'm happy to ship them your way if you'll get use out of them.
I also have two of the AA powered "Gotham" front lights which are heavier, less waterproof, and, if I recall correctly, emit less light. Yours if you have use for them.
That would be greatly appreciated, thank you very much!