A generation ship is made pointless by technological advance. Why leave in a 1000yr generation ship now when it will only take 25 years to develop one which get

A generation ship is made pointless by technological advance. Why leave in a 1000yr generation ship now when it will only take 25 years to develop one which gets us there in half the time? And why then if another 25 years could cut that time in half again? For that matter, why not wait, say 150 years until a combination anti-matter / interstellar-hydrogen fueled engine that could accelerate at 1g for a decade? It'd be kinda silly to arrive 1000yr later only to find the system entirely colonized by people from the 'future'.