Let's walk through your examples:
SQL libraries: Korma is dead, YeSQL is dead, HugSQL is in maintenance mode. The community converged on next.jdbc + HoneySQL. That's not fragmentation - that's exploration followed by selection.
Build tools: Boot had its moment, it's essentially gone. Leiningen is still around but new projects overwhelmingly use deps.edn. That's not fragmentation either - it's a transition, and a remarkably clean one.
Spec vs. Schema vs. Malli is the most interesting case because the convergence is still in progress. Schema is effectively in maintenance mode. Spec has stalled (spec2 never really shipped). Malli is clearly gaining momentum as the community choice for new work, largely because it treats schemas as data — which is very Clojure. But even here, these aren't competing implementations of the same thing the way, say, Javascript build tools are. They have meaningfully different design philosophies, and the community is converging on the one that best fits Clojure's values.
Compare it to JS, where we have Webpack, Vite, esbuild, Parcel, Rollup, Turbopack, and the "winner" changes every 18 months. Or Python's packaging story - pip, pipenv, poetry, conda, PDM, uv - which is still a mess after decades with a vastly larger community. Fragmentation in large ecosystems often doesn't self-correct because there's enough mass to sustain competing approaches indefinitely.
The small community size that you frame as a weakness is actually why the self-correction works. There aren't enough people to sustain five competing SQL libraries, so the best one wins and everyone moves on. The tight community, the shared values around simplicity and data orientation, and the fact that people actually talk to each other - these create selection pressure that larger, more diffuse ecosystems lack.
The productivity gains aren't speculatory - from the most pragmatic angles, I see time and again - they are real and palpable.