[coldtea]

>I notice that Oumuamua happened to pass within some 20 million km of earth within a decade or so of having systems in place to spot it. Wouldn't this imply there are an awful lot of them?

Finding one in a decade's span within 20 million km would imply there are "an awful lot of them"?

[Baeocystin]

Practically instantaneous on the lengthy time scales the universe operates in, don't you think? And our observations have far from complete coverage of the sky.

[azernik]

That's a pretty small volume and a pretty short timespan, all things considered.