There's no need to overcomplicate the situation. Joe Biden is 81 years old and had a really bad debate. And there are some people in and around the Democratic Party who would like a different candidate, because Trump would be an absolutely horrific president. They've been making a lot of noise this week. Because if this were to happen, right now is the time (ie. before the convention).
There are also a lot of people who want Biden to stay, probably a much larger group. But everyone is aware of the dangers of EITHER choice. Nobody has a crystal ball.
These are not deep state CIA operatives, and they're not lizard people. They're just a bunch of politicians, political operatives, and people who work for the party, people who write and tweet about politics, etc. The media is pimping this narrative for their own reasons. It does generate a lot of clicks.
Don't kid yourself that there aren't a lot of people in the Republican Party, especially those with any sense of morality or actual patriotism, that would love to replace their candidate too. There's just no way to do that right now. We can see with the large amount of people voting for Nikki Haley in the primaries that there is an appetite for this. Not a conspiracy, just a bunch of people with opinions.