No, It wasn't realistic at all.
Ron Moore fanboys keep using "Duh, all their technology was falling apart" as an excuse. They fail to realize even with the basic machine tools they had, they would have easily been able to setup a primitive industrial society. With those primitive tools, they would have been able to make more refined tools, and so on. Instead of 150,000 years, it could have been a few hundred years of development. All their knowledge of the mistakes of the past could have been remembered. The "cycle" could have truly been broken.
Instead they descended into neo-ludditism, which basically guaranteed that most of them would die from disease and starvation. How anybody believes giving up pretty much all your knowledge and history* is "making a clean break" is beyond me. It's running away from your past and dooming thousands of people to death.
* You would think that their history is something would probably want to remember, so that they don't make the same mistakes again, but then again this ending was monumentally retarded.
And all the people who completely missed the point of the story fail to realize that Lee and the Colonials decided
that rebuilding a new society exactly like the one they just lost would defeat the point of beginning with a clean slate. Whether or not they caught diseases after the screen faded to black is irrelevant. What's important is that they decided for a change. If you've watched the series at all, the pre-Cylon New Caprica disaster illustrates their struggles in building a new society from scratch pretty well.
It's also blatantly stated by Lee during his bit about human technology moving faster than the human ability to control it. Just because you personally take issue with humans giving up technology after the apocalypse doesn't mean it is "monumentally retarded"