Wood – What kind of hinges

Yes, it will sag. I'd guess anywhere from 1-3 inches over time, depending on the moisture content and grain.

Whether it's still able to resist compressive forces isn't dependent on that, though. It'll fail laterally (on the thin dimension) before it'll fail vertically due to sag. You should use a T configuration or a heavier (or metal) rail.

For 30 years, assuming concrete, I'd go with un-boxed metal u-channel posts. I'd put bevels on the rails. A 45 degree bevel on the top. It should slant away from the pickets to shed water and dirt away from the pickets. And a 15 degree drip-edge bevel on the bottom to keep the water from running back towards the pickets. I'd get full inch rough sawn air-dried pickets from a saw mill, not the 9/16" pickets from big boxes, though there is something to be said for more rails and thin pickets.

And since price/labor is a function of the number of pieces, I'd use (2) 2x6's for the rails instead of (3+) 2x4's.