Plekanec was brought in for one year, nevertheless. He'll help in that department but it's not like he'll show the kids everything in one year and then they're ready once he goes. Someone like Byron can help keep it going after Plekanec is gone again.
Also, signing vets in free agency every year is not the best way to get the kids ready. They'll keep having new "teachers" every year instead of having some stability with some teammates.
And you're right. We don't know if Byron would have come back as a free agent like Plekanec did. But I'm more than fine if the organization decided not to find out. It's not that I was opposed to trading Byron. But I think he fits the type of style they want to play and he's made for today's NHL, even at 29 years old.