Well, in the scenario depicted, it would be the Federation's planets getting nuked.altnameJag said:For the same reason the Imperium is willing to spend millions of lives and centuries of rebuilding to defend the planets it currently has: if they destroyed habitable worlds at the smallest sign of resistance, it'd run out of planets.
Unless the Federation had something the Imperium really really wanted and the Imperium knew about it, there's really no reason why they wouldn't start hurling cyclonic torpedoes around as soon as the naval conflict went south. Assuming they're able to; it may be difficult if they can't locate Federation worlds or simply can't get a ship there safely.
"Win but get weaker" is an oversimplication. Numerous wars in the Imperium's history have resulted in tangible, non-Phyrric gains. Lord Solar Macharius conquered one thousand worlds in just seven years, only a few centuries out from 40k's "present" day timeline. Not "reclaimed"; he literally pushed the border of the Imperium as far as it could go. Aside from that, the Sabbat Worlds Crusade and the Angevin Crusade (off the top of my head) both resulted in the reclamation of vast stellar territories that had been either uninhabited or just isolated for thousands of years.altnameJag said:Even when the IoM wins a conflict, it comes out of it weaker. That's been true throughout its lifespan. On considering the toll in men and materials, not to mention the inevitable rebellions, defections, and civil wars that are going to break out when they come in contact with a Federation that's human controlled, hugely technologically advanced, doesn't run its entire economy based around human rights abuses, and will even let defectors still worship the Emperor, the Imperium, even if they win, are going to pay a hugely disproportionate amount.
The Imperium is decaying, but it is doing so slowly and on a galactic scale. Individual campaigns still result in large gains or the recovery of long-lost territory. As a historical comparison, the Byzantine Empire took nearly a thousand years to fall, and experienced a half-dozen vast fluctuations in the size of its territory and the strength of its armies over that time. It wasn't a steady one-way decline.
And would the Federation permit potential defectors to worship the Emperor? As in...the nominal ruler of the political entity that they are at war with? The guy whose state religion was founded on an extreme human supremacist ideology that makes Khan and his eugenicist buddies look like flower children?
I don't think Starfleet would be okay with that.