The alternative is anything that can help you warm up during the winter, or to cook something to eat if you have no other source of heat. There is a type of tree that grows very fast and I know it has spread a lot in Europe in the last 10-15 years - and it takes about 3-4 years to grow a tree for cultivation. The ash produced by burning wood is a good fertilizer, and it can also be used as a protection against snails if it is sprinkled around the plant. Today, it is also possible to buy smoke filters that significantly reduce pollution - so everything is not as black as many think.
https://en.wikipedia.org/wiki/PaulowniaThe interesting is that people are not realizing "why" firewood is used right now as alternative and why it is a good thing. The real deal is that Germans realized long time ago Russians can't be trusted and Putin is the evil person in the world today, history always had those evil people, maybe Putin is not as bad as Hitler, but certainly the worst for the world today.
So instead of gas, oil, petrol whatever that you can get from Russia, they would use firewood, but not forever neither. Even though as you mentioned it is not as bad as people imagine, it is still not a forever thing, Europe is trying to build self-energy sustainable ways and when that happens, firewood will slowly fade out again.