The wide array of answers here, and disagreements in the comments, only helps when I give you the dreaded answer that nobody likes to hear: It Depends. Everyone has their own opinions about what is important and what is not important.
Fortunately for most of it, what it depends on is you.
Safety (Generally, "Must Have")
There are a very few things that are absolutely essential which you definitely should have for safety reasons, unless you are willing to risk your safety for reduced weight (I advise against it)...
- Fire starting tools
- Med kit, but you might consider leaving out things that won't apply to your situation or which you do not know how to use anyway. Ex: I have a snake bite venom-extraction tool (suction) which is not needed in my area (not to mention many people suggest against using them anyway).
You need to figure out what you are comfortable with, and your weight carried will generally be a weight/comfort trade-off. Only you can decide where on that scale you lie.
If you really like being toasty warm, bone dry, well fed, well watered, and to do all that with minimum effort so that you have more time to sit and admire nature, then you need to get used to weight.
If you really like being highly mobile during the hike, and you are willing to either spend more time in camp-setup or not be fully sheltered, and have other niceties you don't mind sacrificing, then you can get the weight way, way down.
I am on the "travel lite, sacrifice comforts" side. I would be willing to take just a tarp and sleeping bag for shelter. Since I like to camp even through the winter though, the sleeping bag specifically I do not sacrifice on. Cold weather bag under a tarp is fine.
Water: I'm fine taking little to none. Gathering and boiling; for me that is part of the fun experience but for others that might be a nuisance.
Food: I don't mind getting hungry, don't mind eating some edible leaves, or even stripping some inner bark off my firewood to eat before burning the rest. My food expectations are low.
I've been more than 3 days without anything but water, and I've been more than 24 hours without even water. And there have been plenty of times where I've gone a week or two eating only once every day or two days. The 24-hours-no-water was not planned but it wasn't terrible. Going for days on no food or little food is not exactly planned but I knew it was a possibility, and I was fine with it.
I know plenty of people who would refuse those conditions: 2+ square meals per day or they quit.
For fire, I'm with you on backups-upon-backups. I like to have at least 3 different ways to start a fire available, but I'll take even more if I can. I prefer ferro rod to start mine with a spark, but having lighters and matches available is a good thing. I had to use a bic lighter to start a fire once this year when the rain and wind were bad. The backup was welcome.
However, my primary fire kit is just a tiny tin. It's not an altoids tin, but it's similar, a bit smaller I think, weighs probably a few ounces when full of my kit, and it's sufficient almost every time.
I do not weigh my gear, but I think I would be fine with 10 pounds (that's about 5 kg) or less for a 3-day trip. I am sure there are some who would be comfortable with less.
If, for some unknown reason, I could go on a 3-day trip I wanted but could take nothing but a bottle of water, a knife, a tarp, and my tiny fire kit, I would not let that stop me from going on and enjoying said trip.
Also keep the specific trip in mind. If I were to go on a 3-day hot-desert trek right now I would take gallons of water, a few granola bars, and only 2-5 pounds of other stuff. In the desert I would be paranoid about getting stuck without enough water, so I would be willing to carry 20+kg (40+ pounds), almost all of it water, where someone else might bring a battery-fan and a tent.
On the other end of the spectrum, I sometimes camp with people who "need" so much stuff that vehicles must be brought to carry it all and you cannot stray far from the vehicles. Hundreds of pounds of stuff for a few people. Full camp stove, full kitchen set of pots, pans, dishes, folding chairs, folding tables, extra shelter to protect this mobile kitchen from weather/bugs, etc.. For them, carrying as much as they can possibly lift would still not be enough.
Make sure you can supply yourself with the food and water you need to stay fed and the shelter you need to stay alive.
I think people should push their limits and try to live with less from time to time since that only makes us better, but each individual needs to figure out how light they can travel and still be comfortable and happy. Don't go with 2 kg of gear and have a miserable time just because I said it's good for you. Bring enough to enjoy the trip. If that requires 18 kilograms, so be it.
Make your best guess, then keep re-evaluating.
I'll close with a couple tips: An easy way to reduce weight without sacrificing is to research edible plants. If you eat the occasional leaf while you walk and reduce your packed food accordingly, you get a weight reduction practically for free. Second: some items are consumable, therefore some weight is diminishing - 1 pound of food is only carried on day one, 0 - 0.5 day 2, and it's not in your pack at all while hiking day 3, and similar for water reserves.