It's worth noting this leaves you open to security vulnerabilities. For example, iOS 10 has not been patched for Spectre or Meltdown, let alone any of the other security patches that been and continue to be released with every update.
I'm happy to take a performance hit with iOS updates until it's unbearable, then I'll pay to upgrade the device. Getting hacked would amount to thousands of dollars of damage + heavy amounts of stress, so a few hundred dollars every other year more than pays for itself if it lessens the chance of that occuring.
Of course, some people might not value their privacy, identity, personal files, private messages, email accounts, or credit cards. In which case there's no harm in sticking with an older iOS.