I used my Nexus 5 for 5 years with updates, then put LineageOS on it for another year. Then it broke because of a faulty home button switch. However, I could have purchased a replacement mainboard to still keep it running.

More generally, I'm not sure 1.5 years vs 6 years is the correct order of magnitude when talking about environmental impact. Call me back when I can use a phone for 20 years.