It's no secret that Ben (and Theo) are all about building a strong farm system, then filling the holes with free agent signings and trades. A '5 year plan' was never mentioned specifically, but it takes about that long to get the pieces into place.
The Red Sox fan base, on the whole, does not have the patience for a complete rebuild, like Theo did with the Cubs. Ben had to try to keep the team competitive while rebuilding. IMO, he did a pretty good job of that. We won a championship in 2013. Our teams failed miserably in 2014 and 2015, but 'on paper', the teams that Ben put together were contenders. As I've said many times, that's really all you can ask of a GM. What happens on the field is beyond his control.
What I think happened is that Henry 'panicked' after two last place finishes. JMO, of course, but he has shown the tendency to act rashly when things go badly. He should have given Ben the chance to stay on as GM, now that his long term vision has come to fruition. I am very certain that our teams would have done just as well with Ben at the helm as they did or will do with Dombrowski at the helm. This is still largely Ben's team, after all.