Diggs and Higgins are star performers, going by their stats. I've heard nothing about either's contribution to the lockerroom or receiver's room. Whether either has the rapport with his quartetback that Kupp does with Stafford is unknown to me but seems unlikely.
Kupp's numbers last year taken as an isolate, journeyman pro. Is that his value?
Kupp's presence on the field as a threat to be reckoned with, especially in crunch time, invaluable.
Kupp's rapport with his QB and the way they plan and work together to lift the entire offense, Kupp's role as a mentor in teaching Puka what sets him apart in addition to working the entire WR room, Kupp's inspiration and example as a leader on and off the field, his overall value to the Rams when all is taken together - priceless.
His departure, whenever it comes, will create a vacuum.
You don't trade all that value without serious and comprehensive thought.