I don't think doubling the support would have been nearly enough to ensure Ukrainian victory.

The fundamental issue is that Russia has not fully committed to winning the war either. While losing the war would be an existential threat to the Putin regime, not winning it is not. As long as the war drags on, there are more effective uses for Russian resources to ensure the stability of the regime. But if the war becomes an existential threat, Russia could mobilize its entire economy.

A regime change in Russia is the only way Ukraine could win the war. Maybe by a coup or by military force. Or maybe by an arrangement, where the current regime can retire comfortably in a third country without having to answer for its crimes.