He's definitely not irreplaceable... I really don't see how someone could justify saying he is.
I'd still trade him for the rights to 18 year old Brandon Ingram without thinking twice about it, I'd trade him for the Greek Freak straight up too.
The top SF's in the 2 drafts after that specifically Josh Jackson, Jayson Tatum and Michael Porter Jr all appear to be at-least as talented as Hayward.
As for will he or won't he be around?
I wouldn't be shocked if he walked, he'll be 27 so he could definitely be looking at these negotiations like he's gonna want to squeeze every last drop out of possibly the biggest contract of his career...
He's good enough where some team could pull out all the stops to add him, he;s gonna be an ultra-high priority FA and I'd imagine teams are going to try and sweeten the deal with all sorts of extras, they'll tell him how marketable he is and show him all sorts of plans for the future. It;s a slippery slope for Utah..