So the Empire has 799 Maintenance Facilities (plus maybe some in transit). . . that's one-point-six million tons of military craft it can maintain.
. . . and we've seen one-third of that tonnage fail to stop the Tyranids. I think humankind's only hope lays in the slow speed of the Tyranid advance; the Empire can probably discover, exploit, & research enough new resources to defend itself, but I think Taranis will be avenged (yet again), rather than defended.