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More people know TS (or JS, close enough) than Pkl, it can be used for more, and it's got better tooling/support. Those are understatements.

TS tooling won’t tell you when your config value violates the constraint “isBetween(0, 100)” or “matches(Regex(…))”. Among other things.

A config validator, which can be written in TS, would.

Agreed, runtime checks can be implemented in typescript - that’s the beauty of it

In theory, yes. But I doubt that we’ll see a TS IDE plugin that underlines violated constraints with red squiggles anytime soon. :-)

There are IDEs that auto-run unit tests and highlight exceptions thrown in the main or test code, which wouldn't be far off.