Different tools are good for different jobs. You’ll need to learn that, and you’ll need to learn that sometimes a different framework is better for a different job.
For starters you have full-stack frameworks and micro-frameworks. You have frameworks that use the latest PHP 5.4 bleeding edge features and frameworks that hang back and support the lowest common features. CodeIgniter is perfect for building distributed applications (like PyroCMS, Pancake, ExpressionEngine, MojoMotor, etc). To recode it to use PHP 5.3 would not only break THOSE applications (the sources of income for myself, a few friends, and EllisLab) but would also break every single CodeIgniter application out there. Why do that?!
If you want awesome new PHP 5.3 features then as I see it you’ll need to learn a PHP 5.3 framework. Your requirements no longer match the goals of this framework, and that is ok