A lot of things could be done to simplify the amount of labor it takes to build a web browser. If you look at the architecture of the web, there is no reason for things to be this complicated - the web doesn't need a complicated high-level language runtime when it could have bytecode a la WASM, for example. But that's exactly the problem, because the reason why things have become so complicated is not because that's just how complicated the problem space is that a web browser attempts to solve, but because the people in charge of the web actually have a vested interest in things being as complicated as possible. When you already have the budget of a nation state, there is not only no benefit to your job requiring less labor to perform, it actually harms you because now competitors need that many fewer resources to compete with you. That's the real problem - they don't want it to be simple and easy, they want it to be complicated and hard, and the present problem of lack of browser diversity is just a consequence of that.
This is the real reason why things have gotten this bad in the browser space, and it's never going to get any better without first addressing this fact and then by breaking basically the entire web by decimating the volume of standards documents web browsers have to follow. There is no other option. Nobody is going to sift through 114 million words of specification documents to build another web browser for the web that exists today.
The web can't be saved. Its replacement will have to start from scratch.