[tptacek]

Having to find, download, review, and build a bunch of extra dependencies (and surveil all the dependencies you do explicitly want to make sure they're not pulling in fucky code to do table-stakes things that the stdlib could just stdize.)

Again: these are downsides, but not dispositions. On balance there could be good reasons to have a terse, svelte, balletic std. I'm just saying, there's clearly a downside to the decision.