Re: Sounds like Nemesis all over again
No, as I said, do the sums. That is all that is needed for a parallax measurement of a single star. According to the Tycho-2 catalogue here there are 5,227,058 stars between 9th and 12th magnitude. That makes 10,454,116 observations which is obviously a very small needle in a very large haystack.
Look at the list of nearby stars - they were generally detected not by parallax but by proper motion, and parallax only then used to measure the distance to the star in question, i.e. once it had already been identified. The problem with Nemesis is that it would have the same proper motion as our own solar system, and therefore not moving with respect to us, save for a very small orbital movement which to all intents and purposes looks the same as a much more distant star.
As for the detection of that brown dwarf recently that is precisely what I alluded to in my earlier post: you can detect those "easily" by looking for a bright IR source with no corresponding visible source. You can't do that for stars on the main sequence.