Commanders discretion is just that the commander gets to decide which in my opinion is wrong
Any commander worth his 4 bars who would like to extend should only do so after consulting his crew.
I have had times when I was happy to extend mainly because I wasn't that tired , but my FO was on a different time zone and he was exhausted
So a quick how do you feel to the FO came back with an I'm really tired
End of discussion! NO extension very sorry
I'm not about to force anyone to extend if they voice any kind of doubts
I don't care what the reason is . He wasn't happy to do so , That's the end of the discussion