Here's where we agree: You need to stop the erosion in order to fix it. Every developer on the project needs a shared vision of what "better" means. For example, the parent comment mentions React components that manipulate global variables. First of all, everyone agrees to stop doing that, and agrees on the alternative (use props, a redux store, whatever).

But you don't stop the world and rewrite to the new standard. Instead, you apply a strong "boy scout rule" to every code change. If a developer modifies a small component, they also need to remove the global references. If the existing component is large, the developer must extract a smaller one, and the new extracted component must meet the new standard.

This will slow your velocity - at first, by a lot. But you never stop delivering entirely. You never waste time rewriting things that you would never touch again anyway. You also learn along the way - because while "don't use globals" is pretty universal, most architectural decisions are not. You will find that the "clean new standard" changes to match your application. At the same time, everyone on the team learns about the new standard, and buys in to the new way of doing things. There isn't a "new system" with the cool kids, while the B-team strings along the old one and waits to be laid off.