it aint shepards crew, most likely a patrol, specter, or merc group.
also Thresher Maw aint that crazy with how much shit the zerg truly has, what it can do is basically give us the codes for possibly a mutation/alteration of the Nydus worm/cannal. Which case our bases become HELL to invade as we can reinforce pretty much instantly, and with Nydus worms, we can EASILY start the raids into enemy territory at fricken will.
Add in our capabilities of surviving the shit in the galaxy, and the fact we dont NEED mass relays, makes us a nightmare in the waiting for the reapers to grind on...after all the zerg beat the Xel-Naga with sheer force of wave tactics with primitive for us zerg forms from Zerrus...we can do so again with the full might of a reborn swarm.