Nids win. Slow, numberless, one dies hundreds replace it. Evetualy it will come down to the tyranids vs necrons. Neither of which benefit the other with victory or defeat. Necrons need life energy to feed the ctan (which nids don't have) nids need biomass which necrons don't have. They avoid each other. Once the rest of the universe is dead it will become a endless unwinnable battle. 2 nearly unkillable races.
nids are gonna win the galaxy, its just a matter of time since the imperium cant replenish their lost worlds. plus the eldar are rare as it is and dark eldar are even worse. orks will be the second to last while chaos will be eventually wiped out due to the shadow of the warp and no worshippers.