I hate to say it, but an iPhone battery lasting less than a day is "operating as designed." Take a look at the power receptacles at airports. Most of the time, you'll find people with iPhones flocked around them.

Lithium batteries are unstable by nature. Due to this instability, they naturally age quickly, even if unused. After ~5 years, they're already down to ~70% capacity just sitting there. Like any battery, they also will degrade from usage cycles. I've seen various #s quoted, but in my experience, ~500 cycles on the tiny, thin Lithium batteries used in mobile phones will bring the battery's capacity down to ~50-60%. The thinner the plates inside the battery, the more damage they endure with each wear cycle.

It's no secret that iPhones have some of the smallest capacity & thinnest batteries in the industry. Because these batteries are so small, these phones are going to have more issues with wear cycles. Apple's fully aware of this, which is why we had the whole BatteryGate issue late last year. It's also no secret that Apple goes out of their way to prevent people from replacing normally-replaceable parts in their products, hoping you'll go to an Apple store and buy a nice shiny new product than repairing yours. This is by design.

The iPhone 5SE was released in 2016, 6/6Plus in 2015, 7/7Plus in 2016. So, those phones are all ~2-3 years old. Assuming someone's charging their phone an average of once a day, that's 730-1095 cycles, well past the recommended 500 cycle design limit. So, what you're seeing is the product working exactly as designed.

Don't like this? Give your business to a different company, one which has user-replaceable components. I'm still using a Note 4, which I've upgraded to 128GB ($38) and replaced the battery ($12). I've done similar with Ms. KRSW's S5, which also has user-replaceable batteries & user-replaceable memory card slots.