Hard locks attract workarounds, workarounds become a honeypot for malware.
My biggest criticism of macOS Sierra and macOS High Sierra, is the hard locks preventing installs on older iMac 24'' 2008/2009 Models. These hard locks preventing installation, just attract workarounds, and the workarounds become honeypots for malware. Apple's greed is compromising user's Security. There is no need for a hard lock, just a warning during install.
The newer iMac 21'' late 2009 uses virtually the same hardware as the 24'' early 2009 model, A 2.66GHz Core2Duo Penryn Processor with Nvidia 9400M Graphics, yet the early 2009 24'' model is hard locked from installing macOS Sierra / High Sierra. The reason, an incompatible Broadcom Wifi Card. It's annoying because it's an easy fix in terms of replacing the Broadcom 94321 PCI-e Wifi Card with a Broadcom PCI-e 94322 PCI-e Wifi Card, just a straight swap. Or easier, a plug-in USB Wifi Card.
The 24'' 2008/2009 Models also have no direct replacements in terms of screen resolution/quality, so if you're used to that size of screen, or have external monitors to match, the is no easy upgrade solution.
Drop the hard locks Apple. These excellent classic 24'' iMacs don't deserve to end up in Landfill.