There's two very big difference between WWI and the decades since then:

1. The existence of socialism. Socialism didn't exist in WWI, neither in principled nor revisionist forms. Democratic socialism and social democracy didn't exist either, the closest being the Kingdom of Sweden, which acted more like a failed imperialist has-been empire than a progressive force. Successful national liberation struggles that captured state power and were able to expel imperialists from their lands didn't really exist during WWI either, the closest being Liberia and Ethiopia. The Republic of China was controlled by the comprador beiyang government, Iran was also semi-colonized, and the rest of the world can be neatly divided between imperialist powers and imperialized colonies. This is not at all today where you have various progressive countries like Bolivia as well as countries like China which constitutes a form of socialism or at least a progressive force if you don't believe they're actually socialist.

2. The existence of fascism. Fascism also didn't exist in WWI. And there's a qualitative difference standard liberal barbarism as articulated by Rosa Luxemburg and the absolute debased barbarism as demonstrated by rabid fascists. A proven tactic towards opposing fascism is the united front, where socialists tactically unite with various progressive liberal forces to stomp out fascism. Because at the end of the day, fascists should be dealt with by shoving them into woodchippers feet first. They should be thrown down wells and have grenades thrown at their broken mangled bodies to make sure they stay dead. British soldiers charging no-man-lands to kill German soldiers is a tragedy because it's workers killing workers while British soldiers charging the machine gun nests at Normandy to kill German soldiers is an act of heroism towards ridding the world of fascist dogs.

It's the simultaneous existence of both socialism and fascism that greatly complicates the application of revolutionary defeatism. Like seriously, how would revolutionary defeatism pan out during WWII? Do nothing until Nazi Germany invaded the Soviet Union, in affect copying the absolute opportunistic vulture that is the US? Obviously, when your revolutionary theory makes you act in almost the exact same way as the US, it's time to go back to the drawing board and think things over.

As for Russia vs Ukraine, how you evaluate the conflict goes back to my two points. Russia may not be socialist, but Ukraine isn't exactly free of fascism either. At a certain point, it doesn't matter how far from socialism Russia has fallen from if it can demonstrated that Ukraine has been completely consumed by fascism.