Its from a different domain, but it gives you a flavour of the headaches you encounter. These systems always look simple from the outside, but once you get inside you find endless reams of interrelated and arbitrary business rules that have accumulated. There is probably no complete specification (unless you count the accumulated legal, regulatory and procedural history of the DVLA), and the old code will have little or no accurate documentation (if you are lucky there will be comments).
The ONT's job is to translate from (typically) Ethernet to the optical fibre, and nothing else. In networking terms its "Level 1"; concerned only with moving bits from one end to the other. Most ISPs will provide an ONT which does that and nothing else, and then a regular router/firewall that plugs in to the ONT via Ethernet.
Your security barrier is the firewall in the router, plus whatever encryption you apply to comms outside it. As long as you get that right your ISP can't see what you are doing apart from the to/from addresses on your packets (which can't be hidden, obviously).
ISPs generally push their own managed router/firewall at you because that way when something isn't working you don't wind up with arguments about who's fault it is, and the ISP can troubleshoot your router. But in my experience they have no problem with you unplugging their device and plugging your own in instead.
I haven't seen an ISP which does the ONT and the router in a single box. Its theoretically possible, but would be a bad idea for several reasons. One is security, as you say. Another is that the fibre can't be extended with more wire, unlike a copper phone line. So the ONT tends to be a small wall-mounted box with an Ethernet jack in it. That way your Wifi access point isn't stuck low down next to your front door or something.
I think I see what they mean. "Misconception" means something where you can point out "no, that's incorrect". But these people have more than just an error in their knowledge, they are fundamentally misconstruing the world and how it works.
* The time you spend managing and supervising, including desk-checking the stuff they send out, because it goes out under your name and you are only as good as your last job.
* IT costs, and office space if they aren't working from home.
* Liability insurance in case they screw up. (Not certain about this: maybe you just trust that your company is a sufficient legal firewall because its only asset is you).
I remember back around 1990 hearing that for my big company employer putting a coder in a seat in front of a computer cost roughly twice their salary. That's still true today.
How much maintenance does this take to keep its reflectivity? Does it get colonised by algae? If it needs to be scrubbed with bleach once a year, I can see that being an issue.
Has anyone pointed out that in the UK 999 is the emergency number (like 911 in the US). So "999 Request Denied" sounds like a public safety issue to someone who doesn't speak tech. Make it 998 if you must.
I use a Raspberry Pi, cheap USB webcam and https://raspberry-valley.azurewebsites.net/MotionEye-OS/ Motion Eye to watch out for intruding cats coming through the cat flap. Entirely open source, completely under my control, and simple to set up. What's not to like?
https://wiki.c2.com/?WhyIsPayrollHard
Its from a different domain, but it gives you a flavour of the headaches you encounter. These systems always look simple from the outside, but once you get inside you find endless reams of interrelated and arbitrary business rules that have accumulated. There is probably no complete specification (unless you count the accumulated legal, regulatory and procedural history of the DVLA), and the old code will have little or no accurate documentation (if you are lucky there will be comments).