Apple's vintage clock starts when they stop selling something. They sold the 6s until 2018. They rarely sell MBPs for more than a year so usually in 7 years they're done.
I'm glad you're currently well to do and can afford all new shit for your kids. Some people can't and sometimes a repair is the only option. Is it worth making repair considerably harder for a few extra mm of thinness and less than a hundred grams of weight? I'd argue no. Compare the repair of a 2010 MBP with one from 2016 and then tell me the 2016 one is easy.
I've also replaced several batteries in Apple products, even without pull tabs and it's not difficult.
Regarding replacements, my mother's 6s just got a second new battery last month and it's still supported after 7 years.