You can extend the life of Li ion batteries by not exposing them to temperature extremes (don't leave them in cold cars overnight or in hot cars for extended periods). Don't drain them excessively. It may be less expensive to use a iPhone case with an additional battery, or use a 3rd party to replace the battery (Mission Repair, uBreakIfix...). If a case with an additional battery is used from the start, that may extend the life of the internal battery, due to not draining the internal battery as much. I remember when pagers used disposable batteries, and when you could replace batteries on a flip phone. The build quality on those units was not good, and not at all water proof.
I've never needed an iPhone battery replaced. Have had battery on my 14-year old iPod Classic replaced once. Also, the Nickel Metal Hydride battery on my '13 Prius is 10 years old, and still going strong (average life 10-15 years). I live within 55 miles Northeast of Cincinnati, OH. Try to park in the shade in summer.
As far as laws concerning easy replacement, remember what a former US President (#40) said. The most terrifying thing to hear is I'm from the government, and I'm here to help you. Open to the possibility, but doubt it is economically possible to achieve with any degree of economy (would you pay $100-200 more?). If possible, name one other water resistant phone that lets you replace the battery easily.
Given that Google/Alphabet, Amazon and Microsoft had a horrible year on the stock market last year, and Apple not too much better, maybe longer life batteries will be on the way. Many others, including TSLA and DWAC, were down much more. A solid state Li Ion would be good (but likely more expensive). Multiple companies have been working on these for several years, with slow progress.