The quality perception of this user segment is relative, too. That visitor who is using a version of Chrome that is 4 years stale? Their experience on the rest of the web was not pixel-perfect today. Many sites are inexplicably buggy for them. They might even be used to having to switch devices to complete some tasks.
Continuing to surf the web with a long-unpatched browser is also overtly negligent [1]. That negligent 2% of users is not a protected class. As such, you might observe that this 2% of users contributes only 0.2% of revenue / engagement / value and make a self-interested decision to stop supporting them.
[1] Except brief windows of time when old Apple devices continue to get Safari security updates without getting feature updates. Not relevant to the author's CSS nesting example.