Believing Russia will likely win eventually is not a "myth," it's (mis)calculation, and it mostly doesn't come from Russia, it comes from outsiders who look at the population imbalance and the lack of leadership in the West.
Biden was a fool and a coward who allowed Russia's nuclear sabre rattling to slow walk the war materials and rules of engagement until it had enough time to regain its war footing. Trump is an idiot who got all his historical 'facts' from ignorant far right idiots on the internet (and who in turn got those facts from Russian propaganda).
The west has been played like a fiddle from the very beginning of this war, starting with Obama refusing to arm Ukraine because he distrusts American power more than he distrusts Russia.