Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150604030859/@comment-6078851-20150604153410

if there was no heresy, or any similar alternate civil strife, then there would be far less species within the milky way than there currently is :)

one of the many detrimental effects of the heresy was the destruction of the imperial webway project - if that had come to fruition the imperium would have gained an insane advantage. with access to the webway the eldar would probably be extinct by now as the imperium could simply throw uncountable numbers of troops at the eldar, and with webway access they could throw such numbers at maiden worlds and even straight into craftworlds. plus, if emps was still ambulatory with 20 strapping sons at his side, as well as the astartes legions of old (no heresy means no restructuring into chapters), there's very few battles/wars they wouldn't be able to win. if humans gained access to the webway the eldar would most certainly try to prevent their usage of it, which would probably result in an all-out webway-war, which the imperium would win - with low eldar numbers their only advantage in a war against the imperium is the webway, so without that advantage they lose. then once the eldar are done the imperium would also set its sights on getting rid of the dark eldar.

the imperium would still have need for warp travel though: the webway, while providing a safe and faster than light mode of conveyance, is broken and fractured. many parts of the webway are inaccessable, for a variety of reasons, and it doesn't provide access to much of the galaxy - it's for this reason, and that warp travel is intrinsically more dangerous for eldar, that the eldar never travel far from webway portals. but a lot of the imperium is way too far from webway access for the imperium to dispose of warp travel.

if the imperium had survived intact to the present, including emps and sons (which is way too happy an ending for 40k :P), then the only things that would be a threat would possibly be necrons, if they were united, the ever present threat from chaos, and orks - maybe not an explicit threat but orks will always be around to harass :P  tyranids could be a threat but with a galaxy under emps' dominion, a united and unbroken imperium with astartes legions (not to mention the trillions of guardsmen), and webway use (by now the imperium would probably be the sole users of the webway), even i, one who sees tyranids as the death of every species in the galaxy, think this imperium could probably handle them :)