Others could try, but none could do the role like Thanatos could. He had to be the one to step back in and make up for lost time. However, Vegeta has to question him over it, "Do gods tire? Do you need rest?" Maybe he did on the train, but who knows what limitations the train had compared to Thanatos' home. Vegeta hadn't noticed any on himself, but he's also not a god...

However he can picture himself in the situation, Thanatos had a similar stubbornness to do everything himself, and perfectly. To take on so much, that could be destructive. "I know you want to make right what has gone wrong, but keep in mind you are one being, and if you exhaust yourself then you are going to be useless to everyone. They've waited this long, they can wait a bit longer if you need a time to do your job correctly."