Hence Trump saying well I won’t be there so have at it
My money is on Trump abdicating the day before the inauguration, making Pence acting President who will then pardon him, on the final day. Hence he won't be attending because he won't be the sitting President, and Pence will only be acting President.
He doesn't have to be sitting President to attend, it's the norm for all living Presidents to attend if they can. The only question is if he hadn't declared, would the invite have been withdrawn? He's saved JB a dilema
My money is on Trump abdicating the day before the inauguration, making Pence acting President who will then pardon him, on the final day. Hence he won't be attending because he won't be the sitting President, and Pence will only be acting President.