I think it's about the number of developers involved.
If you have a small team of 3-5 people who understands the architecture as a whole and can keep up with most PRs then I don't think a UI framework is necessary.
However, if you have many teams working on the same project and/or cannot keep up with every single PR merged then I think using a UI framework is a must. It forces you to stick to strict rules. Although you will start writing boilerplate code, it makes the entire application much more modular, and easier to reason about.
