>How often does the developer get bored and start to neglect the plugin?
For the most popular frameworks (e.g. Drupal, Symfony, PHPunit), the core ecosystem is (co-)maintained by the framework authors, who are funded through various means - usually consulting/webdev agencies. The "usual" FOSS burnout problem doesn't apply for them.
> However you end up with stagnation and lack of innovation if you pressure users to use a framework because there's a fear of some sort of exploit.
A framework is just that: a framework. You can develop all you want with that framework - I've seen Symfony being used for tiny microservices to the code for a bank's website. You can develop faster and better code because you don't have to re-invent wheels or do tedious interoperability tests for basic stuff allll the time, and whatever problem you encounter, someone else will have also encountered and posted a solution on Stackoverflow.
> It's like riding a bike and forcing the rider not to ride without stabilisers because they may fall sideways.
No. Symfony, Laravel and others are the vendors for the basic bike... you can always bolt on custom parts according to your need and for the PSR-standardized stuff like loggers, you can even choose between multiple different part vendors.