This is something I've been wondering about for awhile, and apparently I'm not the only one. But did Zeno really destroy all of Future Trunks' timeline. We know he's CAPABLE of doing so, but did he? It was never stated anywhere that he did.
In fact, if we looks at the dialogue, it's more likely that he DIDN'T, and actually only destroyed Future Universe 7. In the dub, Zeno says "All of existence go away!" This could have very easily been him talking about Zamasu's existence. And as we saw when Goku went back to the future to pick him up, there were still fragments of the universe floating around, plus Future Zeno's attendants later came to the past. So it's likely that he just erased everything Zamasu infected. In the original Japanese version, what Zeno actually says is "A world like this must disappear". So again, there's no indication that he wiped out the whole timeline. And possibly most supportive of all, is the manga, where Goku said "I didn't think he'd destroy the whole universe". He said universe, as in only the one. So it was never stated or even indicated that Zeno erased all of the Future timeline, only Universe 7.
More on that. When Zamasu became Infinite Zamasu and took over everything, people just assumed that he spread everywhere. But here's the thing, if he had spread EVERYWHERE, then that would have included Zeno's realm, and Zeno would have just erased him then without having to be called. But even if Zamasu had deliberately excluded Zeno's realm in his take over, it still would have been noticed if he had taken over the multiverse. So why would Zeno erase everything, even the places Zamasu hadn't spread? What more, it also gives purpose to Zamasu killing everyone. When he became Infinite Zamasu, he killed off all life in the universe. This would have spared everyone in Universe 7 from Zeno's erasure because they were all in other world, making Zamasu's genocide a blessing in disguise.
So if you actually think about it, and look at the evidence, Zeno did NOT destroy the entire Future timeline, he just erased Zamasu, who had infected pretty much all of Future Universe 7.