[codingdave]

Knowing vanilla JS and old ways of working are helpful to understand the needs that made React and other frameworks come to be in the first place.

If you really want to learn React or something else, then do it - learn React on the side, or try re-writing the jQuery front-end to React. Or try bundling the jQuery app with webpack to learn those pieces of the puzzle. You can even bring React into the jQuery app and re-write one widget at a time. It would increase the app size, so there are definite downsides, but then you could both keep your job and move forward on tech skills, while slowly modernizing the app.

Basically - get creative. This is not a one or the other decision. At the same time, be careful if you do re-write to React - don't put your learning curve directly in production.