Not so much of an assumption. You also have to take into account the speed at which the Voyager satellites are traveling and how far they will get before the heat death of the universe.

This site says it will take about 40,000 years to get halfway (2 light years) to the closest star (besides the sun that is): http://voyager.jpl.nasa.gov/mission/didyouknow.html

There's a bigger chance humans themselves will retrieve them and put them in a museum but I'm happy to be proven wrong!