I think this lib will benefit a lot if It has support of industry standard way of detecting target JavaScript version, the browserslist. If you don't support tools like that I don't think your lib will draw enough attention.

Yes, esbuild can be used as intermediary step, but still I'll have to run Babel.

And you know, if project is small then compile speed is not the issue. So new projects don't have real reasons to prefer esbuild or esbuild-loader over webpack or rollup with Babel.

Compile speed is the issue for large codebases. But large codebases usually have to address wide audiences and need tooling to deliver code that will work everywhere. Their audience often includes people that use older browsers. Browserslist helps to address that a lot.