People have different reasons for not updating. I have a 2010 Macbook Pro that cannot run the latest patch on High Sierra and I have to stop right before the last known patch. Otherwise the machine goes to an infinite reboot loop. Apple doesn't care for a 9 yr old machine. But it is otherwise a fine piece of hardware - use upgradable HDD, battery etc. I still prefer that to my 2016 Macbook Pro that is a piece of shit machine.
FWIW, I just bought a 2015 MBP and it's brilliant. It has the retina screen, old school keyboard, great trackpad "old USB", magsafe, SD card slot and HDMI. It will run the latest macOS no problem.
I hope the battery was replaced? There's a recall affecting the battery for mid-2015 MBPs. The batteries can explode even when not being charged and the system not in use, it's definitely worth complying with the recall (which they make very easy, I did it with mine right after the recall was announced and got mine back within 4 business days).
Not all mid 2015 MBPs are affected. I think it is only some 15". I own a 13" and 15" and neither are affected. You can verify that the battery is good on Apple's website, by checking out your MBP's serial.
It makes me sad that everything you wrote including the swearing is justified. If only at least one other manufacturer could come up with a decent trackpad.
