This is like a bad navigation virus, spreading from pilothouse to pilothouse, and it really needs to stop.
I agree that AIS should, in general, always be on. But if my mate ever tried to justify his or her failure to detect an otherwise-visible and/or radar contact with the bullshit excuse that they weren't showing up on AIS that mate would be very lucky if all I did was make them take a time-out at the Navigation Re-Education Camp for Incompetent Seafarers. My inclination would be to fire them, and I f that sounds harsh it's surely meant that way.
Incompetence? Laziness? Inattentiveness? There is absolutely no excuse for this attitude to be accepted or allowed to spread. None. However, I'm well aware that it exists. So shame on us all for collectively mis-using the technology that is supposed to help us avoid one another.
AIS can fail at any time. It's just another radio device. It's range is sometimes more limited than what it typically is, and "contacts" will at times drop-out without warning.
Dependence on AIS for primary or near-primary collision-avoidance is a clear indication of a serious competence or attention-span problem. That's a red-line-in-the-sand issue for me. Radar/ARPA remains primary, and for good reason: you control your own radar, independent of what the other vessel(s) does or does not do, or how well (or not) their equipment is working.
So the Crystal does not get a pass in this case just because the Fitzgerald was running without AIS.