Yes, the decision to rely on modern CSS is more like serving restaurant food that contains cilantro. The predominant failure mode is partial credit.
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.