The Imperium needs to have an actual, proper crusade against the Tau, lead by a primarch, and they don’t retreat to deal with something else, they just play out the war to its end. The Imperium bloodies the Tau but they also lose the war and are forced to retreat after a grinding front, no “oh we have to fight the Tyranids now” excuses, they just straight up lose. GW is obviously gonna continue making the Imperium more good and the Tau more grimdark so they gotta make the Tau into an actual threat if they’re gonna go that route.