Scarcity of some knowledge. Not all knowledge exists on SO. You are right about the popular stuff, but the niche stuff will be like everything else niche, harder to get and thus more expensive. COBOL is typically used as an example of this, but COBOL was at least documented. I am saying this, because, while I completely buy that there will be executives who will attempt to do this, it won't be so easy to rewrite it all like Jassy from Amazon claims ( or more accurately, it will be easy, but with exciting new ways for ATMs, airlines and so on to mess with one's day).
<< The person paying would consider whether they'd get locked in
I want to believe a rational actor would do that. It makes sense. On the other hand, companies and people sign contracts that lock them in all the time to all sorts of things and for myriad of reasons including, but not limited to being wined and dined.
Again, I think you are right about the trend ( as it will exacerbate already existing issues ), but wrong about the end result.
why would it be lucrative? The person paying would consider whether they'd get locked in to the framework/language, and be held hostage by the creator(s). This is friction to adoption. So LLMs will make popular, corporate backed languages/frameworks even more popular and drown out the small ones.