Incoming super opinionated post...
Run. Far away. A company that actively maintains "legacy" code instead of investing time in a better/modern option shows a lack of understanding regarding the "developer happiness > valuable product" continuum.
Do they show interest in upgrading their tech? 🤔
Do you want to work for a company that is years behind the times? 🤔
The answer to your original question is no, it is not worth it. Its 2017, and Backbone is dead. Learn something valuable like the other options you mentioned.