So I actually work in Post and this is something I bring up all the time to our company. Grading on these high end displays (we have the 43K Sony) the consumer never sees what they see which is exactly your point. WYSI'NOT'WYG. This is the huge issue with all these color profiles, different displays, etc. Now you do not want to go with the lowest common denominator though. You have to grade for the best and use the best grading monitors.
But yes you are 110% correct that what is referencing even matter if the majority aren't using what's considered a reference.
I'll give you a great example...we had a client in grading her indy film & when she took it home on a drive to watch it, she complained it didn't look good on her home LED LG TV. They went through multiple iterations until she finally brought in her personal TV into the suite & said to grade it according to that.
This is where Apple went wrong...and I was telling my color guy this...
Apple should have fashioned their XDR display/color accuracy as the best monitor for everyone using THEIR APPLE devices...you can guarantee that the color will be accurate from Post, to your iPhone, iPad, iMac, MacBook, etc. if you grade on this thing.
You'll have an eco-system where what you see on the XDR is what you see on your Apple device which is where most people are consuming now.
They also should have come out with a 4K & 5k versions with the 6k that were much lower in price point.
no offense, but that client sounds like the problem in this case rather than anything.