It would be a pain for developers, but firefox and chrome using a gig of ram to view webpages and play videos is horrendous even with isolated design.

That can’t be helped. Hard to explain well without knowing how much CS you’re familiar with, but basically in order to guarantee security/user safety you have to sandbox each tab (basically running an entirely separate program for each tab), all separately running their own interpreters for javascript/typescript, HTML, CSS, all of which are very resource intensive (mainly javascript/typescript). There’s not really any getting around this, no matter how well you design your browser.

Now, theoretically, with the growing popularity/advances in WebAssembly, and increase in usage of frameworks/graphics APIs like WebGPU, you could completely get rid of what we think of as “sandboxing” and completely get rid of the extremely slow javascript and html/css, in favor of completely using safe, compiled Rust programs. There’s active research using versions of WASM which only accept completely safe code (mainly safe Rust code) so using memory bugs generated from user error to access data in different tabs becomes impossible (aside from potential unaddressed bugs in Rust itself obviously) and you don’t need to sandbox each tab. Then you could potentially have browsers with thousands of tabs perform perfectly fine, assuming each of the websites is programmed competently.

But that’s not going to happen, because billions of users rely on HTML/CSS and JS, and it’s not pretty to transition away from. Getting rid of it would be like getting rid of pointy shoes, or getting rid of US Customary Units in the US, it’s just not happening no matter how much benefit it would bring.