I'm plenty familiar with "the metal", and that's all I use for my (simple) personal blog, but I happily reach for React or Next.js when they're right for the project constraints. It doesn't have to be either/or.
That said, similar to GP, I'd definitely recommend it. Browsers give you a lot out of the box now, and life without builds is sweet - fast iteration time, perfect in-browser dev tool support, and vastly reduced codebase complexity.
It doesn't reduce the actual code complexity at all, in my experience. You don't have to deal with configuration, which is nice (though modern frameworks and tools are simplifying that more and more), and you don't have as many dependencies, which can be a positive thing.
But your actual code - where you spend the bulk of your time in medium to large projects - is not going to be made simpler by avoiding dependencies, especially build tools. The app I work on at my job (which is a very average one in many ways) would be a nightmare of complexity without our frameworks and build tools.
There can be this purity mindset that arises around the raw web. It can be fun to indulge for personal projects, and raw web technologies can even be the right approach for some projects. But don't let the siren's song of purity blind you to the messy reality of real tradeoffs.
It's always paid off for me to question that assumption - to try extracting the essentials of the app without the framework, as a weekend project or whatever. Often I can cut thousands of lines of code overall and get a faster more maintainable result.
Simplicity is always a virtue and we should always look for good opportunities to gravitate towards it, it just has to be balanced against other things (as everything does)
That said, I really doubt you cut "thousands of lines of" your own code by re-implementing your own partial framework. Maybe thousands of lines of dependencies were replaced with fewer thousands of lines of your own code, but that's a different question
Anyway, I was mainly pushing back against the original comment which seemed to be suggesting that build tools as a category are unnecessary, and that we should all just reject modernity and go back to living off the land. My point was that this is naive, and that it dismisses the very real value people get out of these tools every day