It has huge assumptions. Namely the assumption that the current problems to colonization can be solved in a cost-effective manner.
The longest continuous length of time spent in what limited space flight we have is just under 440 days. Not to mention, all of it was spent in low Earth orbit. Once we go beyond that, we lose all protection from the Earth's magnetosphere.
Then there's also the loss of bone density due to lack of gravity. 1% per month is nothing to sneeze at.
We are not built for space travel. We are built to inhabit this planet. We have evolved to the conditions here.
And before you say that the generation ships will just fake gravity and have appropriate shielding. Where do you get that assumption? We can't fake gravity. We have a space station where they take 10 times the radiation as here on Earth because we can't even realistically shield that.
And then we get to "autonomous probes", the great handwave of people who think they can cleverly sidestep the "humans aren't built for space" issue.
Who builds them? For what purpose? What's our expected return? How can you justify the investment in time and resources to shoot into space, with absolutely no hope of even knowing there's a chance for a return on that investment until generations later, a probe that can at best return data.
And then there's the issue of it being an autonomous probe. Two words that hide a lot of other problems. A lot of other assumptions. One being that we'll have created an intelligence capable of running this thing.
In other words, the Fermi Paradox just assumes that all of these incredibly hard problems can and will be solved and that the solutions are logistically viable.
What if they're not?
