At the end, with the big reveal that Sumika was the reason for Takeru's stay in the Unl/Alt universe, they went over that it was only possible because of the extreme gravitational disturbances caused by the G-bombs -the very same thing which makes it almost impossible for anything to grow near Yokohama base- and a strong will utterly focused on something; in this case, Takeru.
Given those conditions, since Takeru is still connected to the G-bombing of Yokohama via Sumika (albeit a fading connection) and at the time, he had an extremely strong will, it's possible that Takeru has taken Sumika's place; in other words, his will to save everyone is now the reason that Takeru is trapped in the Alt universe.
... TTGL anybody?
Anyway, that's a possibility. Another I actually remembered from reading Missing: Kamikakushi no Monogatari by Gakuto Coda. Good book, BTW, but here some some spoilers for it. Long story short,
In Alternative, there were only two primary observers left when Takeru finished his journey -Yuuko and Kasumi. While the rest of the surviving Valkyries might count as well, they also didn't have as much of a connection to Takeru as the rest of the girls did. In Paulxion's fic, the whole of Japan at least knows about Takeru, plus a lot of people outside of it in the upper echelons of the UN. And who knows where else the news has spread his name? Even with weak observers, with the tide of numbers on his side... that might be enough to keep Takeru there, especially if 90% of the main female protagonists are still alive, along with closer relations to the rest of the Valkyries whom he's forged deeper relationships with.