I.e. there are always unknowns outside of any model. Arguably this is the only rational assumption you can make: that you don’t know what you don’t know.
Discussion
Your model relies on an assumed continuum of past events. In the grabby alien example it goes so far as to assume a model of the universe which we cannot explain “errrrr dark matter or something”, its practical to do so but to claim a precision to the extent of “no unknowns” I think it verging on hubris.
I agree with this, but you can still make assumptions. Again: your thinking works in the environment where the assumptions are valid and then you only argue that they are reasonable, but of course they are not true.
It's better than random blabbing, which might be true, but you cannot evaluate the situation in which it's true.
I wrote a book about thinking in uncertainty, hopefully I'll translate to English too and you can cover also unknown unknowns, that's not such a big problem.
Another good example is Wolfram's physics. He thinks about all the possible universes with all possible inference rules and he still can make useful predictions about reality, even though it's super general.