Economically, Donbass is critical to Ukraine. Before the war, Donbas provided 75 percent of Ukrainian coal, 25 percent of exports, and 15 percent of fixed capital investment was concentrated there. It was the most industrialized and one of the most densely populated regions of Ukraine. Coal from Donbas is critical for Ukraine's electricity generation. There are a total of 15 thermal power plants in the country, all of them coal-fired. Because of the war, infrastructure has been destroyed and a significant part of the Donetsk railroad - the largest of the country's six railroads - has been lost. Therefore, delivering coal to thermal power plants is now the biggest problem for generating companies.
The regions of Kherson and Zaporizhzhya are no less important; much of Ukraine's agricultural production and the best agricultural land in Ukraine are concentrated there in the south.

As for a "fair referendum", the answer to that is already in my previous comment. There cannot be (and will not be) any referendum without the return of the population. It will take years and decades to rebuild the civilian infrastructure and return refugees and forcibly displaced persons. By that time, the national, linguistic and cultural characteristics of the population will change radically. It is necessary to realize that Ukraine of the 1991-2014 model will never exist again. The new post-war Ukraine will be a completely different country, with a more homogeneous, consolidated society, similar to any mononation of central Europe.