A very valid question. In this case IIRC this drop-off in low-energy solar wind particles was correlated with observed changes in the magnetic field and also an increase in higher-energy cosmogenic particles all around the same time. These three phenomena (observed by different instruments) were theoretically predicted to occur at the heliopause transition. So it lends much more confidence to the interpretation of the data.
I believe that the direction of the magnetic filed didn't change, contrary to expectations. The explanation was that the galaxy's magnetic field is aligned with the magnetic field of our sun.
And hopefully Voyager 2’s measurements will confirm this. I don’t know how long we need to wait though.
I am guessing the number comes from multiple systems of independent sensors. So the assumption is that they won’t all fail at the same period in the same way