Game Engines and Software Engineering

• I tried to find usage statistics but couldn't (perhaps that's a good piece of research).
• There are clearly a scad of engines, and many seem to be under constant development.
• This leads to a host of software engineering questions.
• First, how do you maintain your code when there are new versions of the engines always coming out.
• Do you stick to a version and only occaisionally up date or do you always try to stay with the bleeding edge?
• Second, what is part of the engine and what isn't.
• If I make a new game, do I start with the engine, or an old game?
• More numbers could come out but it's all a question of how to manage large software projects. That includes several different games, and a range of middleware.
• Note that having hundreds of different engines shows that the state of engines is in flux. There's not one (or 6) that are the best.