The reason they created the list was to help their members be prepared for accidents and emergencies. Could you take more? Of course, but these are the bare "essentials."
The original list has been updated, modified and expanded by several other organizations through the years. Here is my version:
The modern approach to the Ten Essentials is to view each item on the list as a "system" rather than an individual object. Each system is included to address a specific need or situation.
Your Navigation System can be as simple as a map and compass, or as high-tech as a GPS and a wristwatch with barometer, altimeter and compass features. But even if you do have the fancy electronics, I recommend also carrying a compass and a topographical (topo) map. Why? They don't weigh much, they don't need batteries, and they can perform other functions. The sighting mirror on the compass can be used as a rescue signal mirror and the back of the map can be used for taking notes. Whatever you decide to take, make sure you know how to use it!
Your Sun Protection System should protect your whole body, so it should include sunscreen, lip protection, sunglasses and the right kind of clothing. It's surprising how little sun protection some fabrics give. Use clothing with rated sun protection or put sunscreen on all over before you get dressed. For sunscreen, I prefer using a sunblock with at least SPF 45 that blocks both UVA and UVB. But both of my parents had skin cancer, so I may be paranoid. If you're lucky enough to not need prescription glasses, get yourself some decent sunglasses that block 100% of UVA and UVB, and that give your eyes good protection from wind and side glare. If you wear prescription lenses, either get good prescription sunglasses or consider wearing the kind of sunglasses that fit over regular glasses. Fashion follows function.
Weather conditions can change quickly. You should take the right type and quantity of clothing to help you survive the worst weather ( cold, wet or both) that you could realistically experience. Layering is a great strategy. Unlike cotton, synthetic fabrics insulate when wet. We lose up to 70% of our body heat through our heads, so a good, warm hat can give you more warmth for its weight than any other article of clothing you could take.
I prefer LED headlamps for outdoor illumination. Why? They leave both hands free for other tasks. Their light is always pointing where you are looking. They are compact and light weight. Batteries last a long time with LED bulbs. LEDs don't burn out or break easily, so you don't need to carry spare bulbs. Most have an emergency flashing signal mode that conserves batteries even more and can be seen at a great distance. But even with an LED headlamp, I always carry spare batteries.
If you're a medical professional, you are competent to assemble your own first-aid kit. The rest of us should rely on a pre-packaged kit that is intended for the type and length of our trip, and the size of our group. Most kits are extremely well equipped, well organized, and include the little necessities that we wouldn't think of, like a medical emergency guide.