This is where ES Modules come into their own and why importmaps are very much an ESM thing, predominantly. ESM was built with "natural" tree shaking in mind in a browser context. Only URLs that are actually imported in a module are loaded. (importmaps are not prefetch maps.) Modules themselves are loaded in a such a way that unused exports in them may be lazy-JITted "weak references" and easily garbage collected (a form of tree-shaking).
You get better "natural" tree-shaking from that library of lots of small little ESM modules, you don't need to rely on maintainers building their own minified bundles.
The obvious trade-off, of course, is that HTTP 1.0 wasn't optimized well for lots of little files and even HTTP 1.1 servers haven't always been best configured for connection reuse and pipelining. Bundling is still sometimes useful for a number of reasons (whether or not minification matters or compile-time treeshaking makes a noticeable difference from "natural" runtime treeshaking). Of course, all the browsers that support importmaps and script type="module" all support HTTP 2 and most support HTTP 3 and that trade-off dynamic shifts again with those protocols in play (the old "rules" that you must bundle for performance stop being "rules" and everything becomes a lot more complex and needs "on the ground" performance eyeballs).
You get better "natural" tree-shaking from that library of lots of small little ESM modules, you don't need to rely on maintainers building their own minified bundles.
The obvious trade-off, of course, is that HTTP 1.0 wasn't optimized well for lots of little files and even HTTP 1.1 servers haven't always been best configured for connection reuse and pipelining. Bundling is still sometimes useful for a number of reasons (whether or not minification matters or compile-time treeshaking makes a noticeable difference from "natural" runtime treeshaking). Of course, all the browsers that support importmaps and script type="module" all support HTTP 2 and most support HTTP 3 and that trade-off dynamic shifts again with those protocols in play (the old "rules" that you must bundle for performance stop being "rules" and everything becomes a lot more complex and needs "on the ground" performance eyeballs).