The issue isn't signing him at the time of his contract. It's doling out that money for a 5'9" point guard who will be in his 30s by the end of that contract, and who relies on speed as his main way of scoring.
It's why I (personally) would love to see one of the young guys like Smart or Rozier develop into a starting caliber point guard. That way, I truly believe Isaiah would take one for the team if it led to more wins (imagine an elite Smart starting and Thomas backing him up!) and settle into a true 6th man role., especially as he transitions to the latter years of his career. That way, his ego is in check, and he hopefully doesn't demand that big pay day.