I'd argue the lack of backward compatibility is what is keeping hosting companies to stick to arcane versions of PHP despite numerous newer releases being aroun

I'd argue the lack of backward compatibility is what is keeping hosting companies to stick to arcane versions of PHP despite numerous newer releases being around. Without a backward compatible environment lots of people will refuse to move forward because of the costs associated with it. For instance Windows would hardly be as successful as it is if it weren't for backward compatibility. Not that it is all golden though. Java suffers majorly from backward-compatibility which has caused the language to be almost stagnant. In this respect I think Microsoft made a much better decision for .NET by enabling the user to have multiple runtimes installed.