Having engineers specialized in a specific stack learn on the job for the sake of/while working on a project is a great way to end up with really funky code and poor user experiences. I speak as a developer tasked with doing exactly that several times at a previous job, and the long-term is never pretty. The first code you write is immediately legacy code, but if you're learning as you write for a project that is already in motion, you're usually stuck with that legacy code until someone goes out of business or the rapture comes and they have to do a reorg because half the team was hauled to the kingdom of heaven, and now there's an MBA running the department who doesn't like you and wants to leverage AI to do the block chain.