Before SQL became an industry standard, many programs which required a persistent store used things like ISAM[0], VISAM (a variant of ISAM[0]), or proprietary B-Tree libraries.

None of these had "semantic expressivity" as their strength.

> If SQL looked like, say, C#'s LINQ method syntax, would it really be harder to use?

Yes.

0 - https://en.wikipedia.org/wiki/ISAM