The only reason they couldn't do it without him was because he knew things that neither Sam or Dean were aware of. Once he dispensed that information, he would be yet again relegated to the status of a 'baby in a trenchcoat,' to use the exact wording. Despite the fact that he'd not worn that overcoat in months and had no intention of doing so with his vessel inability to regulate its homeostasis.
"It's not the demons that concern me," he pointed out. "In any equation, it's always the angels. And they will not stop. Once we close the gates to both Heaven and Hell, that will likely trap members of both species on Earth...and they will pursue the ones responsible."
With it only being him, Cas was fairly certain he could hold out long enough until his siblings' Grace faded. But now Sam was signing Dean and himself up to untold horror. He hoped Sam was aware of that.