I cannot think about future upgrades at this point. My MBPs usually last me at least 5 years, and there will always be a new shiny thing coming. The rMBP from late 2013 is the one currently getting replaced, so I'm not particularly worried about updating or the NEXT FANTASTIC FEATURE coming up. I am upgrading after a long life of my MBP and will not upgrade again until it becomes necessary, for whatever reason. It's my way, to try and pace myself (I'd be buying every new thing if not, to the detriment of my wallet!). iPhone, Apple Watch and iPad, I update every 4 years. Laptop 5-8 years.
That's about perfect! Same with me. Replace when software support ends, but iPhone usually around 4 years and laptops about 6-7. I replaced my 2011 in 2016 as it started developing graphics issues and speakers went bust. Otherwise, it would have lasted me a year more at least.