Agreed, I'm still on a 2015 model and still super happy, even battery life is OK.
Every time a new MacBook Pro is announced, I'm a bit excited but then I compare and I find no good enough incentive to upgrade yet.
I'm secretly hoping that my 2015 model dies so I can upgrade to a newer sexier one, but the damn beast keeps on working great.
I've looked, out of curiosity, at the prices you pay for refurbished ones around here (Germany), and most offers for the refurbished 2015 now exceed what I paid for a new one back in 2015 (same configuration, of course).
I'm still using a 2010, and while I did upgrade to an SSD and added some RAM, it's still going strong. The only thing making me consider upgrading is that OS X upgrades stopped supporting it.
On the 2010 models upgrading the RAM or hard drive just involved a specific screwdriver to take the back off. It took approximately 5 minutes to do at home.
Pretty sure its impossible to upgrade the Air unfortunately.
Also a 2015 MBP user chiming in. I see absolutely no reason to upgrade. Currently in need of a battery replacement. Still deciding whether it's worth attempting myself or to just shell out the extra $100 for Apple to do it.
