I like the "invention wavefront" analogy, but I unfortunately can't remember where I learnt it.
Imagine the high-dimensional space of all possible things that can be invented. In the "centre" of the space is a hypersphere containing everything that has been invented to date. Every new invention is invented by "poking outwards" in one small place on the boundary of this hypersphere (hyperglobule) so that the hyperglobule now contains something that it didn't contain before. All inventions that could possibly be invented right now exist on the surface of the hyperglobule. Think of this as the "invention wavefront". Notice that as the globule expands, the wavefront becomes larger, not smaller. What this means is that as more things get invented, more new inventions become possible (intuitively: you can't invent better brakes for a car until you've invented a car, and so on).
"We might have invented everything" would imply that the space of all possible inventions is bounded, and we're running up against the edges of the box. But I don't see any reason it should be bounded.