Provavelmente, os produtores o levaram para longe do cenário para que não precisassem explicar o que acontece com ele. Eu tenho uma teoria, mas pode conter spoilers:
But I think Deke was primarily responsible for breaking the time loop. What if his return to the past was a random event (as it seemed) and happened only in that timeline, in which the time loop was broken? Deke made his contributions since his return to the past, but that events could somehow have occurred without him having gone back to the past in the previous timelines, there are several moments that he seems useless in the past. The ending would be very similar without him, Coulson did not take the serum also in the other timelines (he does not appear in the flashbacks of Episode 8), but with Deke's speech to Daisy about team unity, changed the way that she and everyone acted (with Mack as the new leader and Fitz and May set out to rescue Polly and Mack) and, perhaps, influenced her speech to the Talbot, which resulted in an event that she locates the serum left by Coulson in his glove, opening up an opportunity she might has not had in the other timelines. In the other timelines, Talbot absorbs Daisy and destroys Earth, the other team members blame her for it, because of his decisions, and the team struggles to work together, as shown in the flashbascks of episode 8.