The Orokin as a society died by the Tenno, the people are confirmed by Simaris Lore and the major story quest that they are alive as they are simply humans. Corpus were a subculture derived from the Orokin that existed before the Collapse and it's hinted that there are other subcultures that survived the Collapse as well, they didn't all live in one spot to get slaughtered. Tenno "killed" them by destroying their entire Executive government in one attack, Stalker Lore explains that when the war was won almost everyone that was important (Emperor, his Vassals, Retainers and other Executives) were all gathered for a big display and killed before anyone realized what was happening. The Collapse that followed these events was made of a combination of disasters, the Grineer Rebellion started, the Infestation had just taken root in major cities and with no remaining upper government to maintain control everything else fell apart. So far from what's known the Collapse wasn't a slow process and it's not explained very well as the following Dark Age hasn't been explored much in the story.