<assuming that it would take 100+ years to get information back from the probe, how do you know that technology 20 or 50 years after launch wouldn't make solar system based sensors able to give you similar information, making the probe largely useless?

Which is why I think no one will ever send a probe anywhere, that would take more than 50 years with technology of the day. We may send a TAU probe (550-1000 AU) when velocity of 20 AU/year or so becomes practical. When 200 AU/year is possible, a probe might go to Oort Cloud (10,000 AU = 1/6 light year). Only when 8-10% speed of light is possible, will anyone send a probe to the nearest star.

Generation ships are a different matter - I think if they ever happen, they will be crewed by the kind of people who do not care about those staying behind, and in fact do not ever want to hear from "decadent, sinful Earth" again. But that's another discussion...