A Russian propaganda win is a win the sense that it enables Moscow to start a new war in a near future. If the Russia can make the 300 000 lost human lives look justified to its own population, it is able to make a micro-invasion into a NATO. In such a scenario, it takes the city of Narva and stops, hoping that at least Germany will refuse to help Estonia regain the city, thus functionally ending NATO. Even if Germany will turn out to help Estonia after all, the terror the people of Narva will have to endure in such a scenario will be horrible, and it also means there is a war between nuclear powers, which is a thing I'd like to avoid. And of course: Refusing to defend Estonia would not avoid the war, but helping Ukraine block the Russia from getting a propaganda victory does.
The Russia must be prevented from getting a propaganda win, because if it does, there will be a war in EU.