The reason ‘Sam has to be the one to stop the darkness’ is because Dean literally cannot defeat her. He’s admitted he can’t. He lets his guard down when he’s around her. She had a hold on him. He literally cannot defeat her. That’s the reason it has to be Sam. Sure Dean can and will help but in the end it’s going to be Sam who defeats her.

deanisdarkness:

Your ask boil downs to this- Dean said he can’t defeat her, Dean
can’t overcome her influence and only Sam can defeat her.(btw nice
circular reasoning you have going on here with your repeated use of the
phrase “Dean literally cannot defeat her”. But I’ll give you the benefit of doubt there.)

Dean said he can’t defeat her –Dean has zero self-esteem. He
also has a very shitty support system. So its no surprise that Dean
doesn’t believe in himself. So you can’t take Dean’s words as truth
here- especially when it’s known that Dean often thinks very little of
himself. Also, Dean admitted to his “weakness” regarding Amara after Sam planted that seed of doubt in his mind in episode “Love hurts”.

Dean can’t overcome her influence–

Dean is bound to Amara, yes, but Dean stabbed her, tried to kill her,
and if she is still alive – its not bc of Dean’s lack of trying.  
Actions speak louder than words.

Dean overcame her influence in mid-season finale so there is no reason
to think that he wouldn’t be able to do it again in future.

Dean
bore the mark, he was the lock and the key to the Darkness’ cage and
this places him in an unique position. If the bond is a two-way
street(there is no reason why it can’t be) then, it also means that Dean
can influence her in some way- this could work in his advantage. Sam has no connections to the whole Amara SL and has no advantage whatsoever.

Only Sam can defeat her –If Sam can fight devil and win, then
there is no reason why Dean can’t do the same with darkness. Dean was
shown to be far more stronger than Sam ever was, Dean was also an
archangels true-vessel, so it would fit if Dean fought and stopped Amara
with Sam’s support- reverse Swan song style. Sam doing another
swan-song when Dean sat sideways- would make terrible storytelling.

So to conclude no, Sam doesn’t have to be the one to stop the
darkness. The show could easily go with Dean being the one SL, for a
change. But don’t worry, with the
writers we have, and with the blatant Sam bias on the show, you’ll
probably get your “Savior Sam/helper Dean” finale.