As long as I am in the world, I am the light of the world.John 9:5
Then what? Global darkness? Definitely not. The disciples did not become orphans (John 14:18). They were going to have – incredibly generously- more with the next phase. Of that phase John would write:
You are of God, little children, and have overcome them: because greater is he that is in you, than he that is in the world.1 John 4:4
Light prevents stumbling. Light departs and sorrow sets in. There is no sin lurking in the shadow of sadness. Lot was vexed daily seeing the decadence and wickedness of the residents of Sodom and remained a just man. When Christ the Light departed light would not become scarce for the disciples. The world would have get drawn to Christ because of his death. The very prospect of darkness saddened the disciples.
5) But now I go my way to him that sent me; and none of you asks me, Where go you? 6) But because I have said these things unto you, sorrow has filled your heart.John 16:5-6
And you now therefore have sorrow: but I will see you again, and your heart shall rejoice, and your joy no man takes from you.John 16:22
The departure of Light was going to lead to a greater provision. The Holy Spirit would bring guidance into all truth that would fill the disciples with light and joy.
16) And I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever; 17) Even the Spirit of truth; whom the world cannot receive, because it sees him not, neither knows him: but you know him; for he dwells with you, and shall be in you. 18) I will not leave you comfortless: I will come to you.John 14:16-18
Sorrow cannot be a good thing even though it comes naturally, and while it does not need to be anyone’s regular companion it is not a sin to be sad. The access to joy and light is wide and user friendly.