Vanilla JavaScript makes sense for personal projects, but if you're working on a team, I wouldn't trust other team members to not create their own frameworks that may not be as well documented.

Especially nowadays with LLMs, the team would benefit more from the LLM innately knowing a widely used library/framework than having to spend context each session teaching the agent your custom setup through context files and skills.

Sure, when I wrote JavaScript for a living I wouldn't trust my employer to get hiring or training right either.