Why do browser vendors have such a control over browsers? Can anyone make a dll injection and bring back manifest v2 ?
They have control over what they distribute. Chromium and Firefox are both open source and people can (and should) use derivatives they find acceptable.
People can create the world, but they do need a lot of time. A lot of money buys you time, so nobody 'poor' can keep up. That wouldn't be a problem for web UAs, if the standards wouldn't be 'improved' at the same path as well. It's kinda like proof of work, but for standard bodies and ownership of a system.