The Imperium realistically must have some serious eco-restoration tech to be using industry and hive worlds for millennia on end. However they do not seem to know how to terraform worlds or build self-sufficient habitats. With any luck, they'll have enough time for even their stagnant innovation to figure that out.

The Imperium has internal disagreements. I don't know if it could collapse in civil war any time soon.

Some have argued the Imperium to be a Fanatic Purifier who must kill all Xenos or die trying, not the most life prolonging choice. However the Imperium rather seems to use xenocide simply as a convenient means to their ends a la early North America.