15.200 Space Marines is quite an impressive amount, but taking into account that both Ork and Imperial Guard numbers ranged in the billions, even that number would have barely been a blip in the Augurs if they weren't that powerful. If that were the case, Space Marines wouldn't be irrelevant on a Galactic scale, they would be irrelevant on a theatrical scale. Warhammer 40K often works on an impossibly huge scale. If a planetary invasion force numers in the tens of millions, which would be the bare minimum to engage in planetary warfare against all but the more sparsely populated Agri-Worlds, taking out, say a few thousand along a few high value targets in a single operation is still a scalpel surgery more than a broadsword swing.
Space Marines are definetly not immortal, but they're very, very difficult to kill in the fluff. Otherwise the Traitor Legions would have run out of Legionnares millenia ago, Warp time or no.