Actually, I've always thought that the main reason MT got overtaken by WP was that PHP is better generally more user-friendly than Perl ;) If it hadn't been wordpress, it would have been some other PHP-based platform. Budget webhosts were more likely to offer PHP support than Perl, and to offer automated installation of PHP software. The licencing screwup just speeded up the process.

Therefore if another framework were to overtake PHP in popularity, wordpress would be dead in the water. I have no idea when that will happen but I find it difficult to believe that it never will. Anyway, I have my suspicions that either #4 or #5 will do for them before that happens.