Forks are not easier to change in a threadless system. There may be a greater variety in replacements(for reasons I already stated), but they both get installed in a similar fashion.

And ease of adjustment? With a threadless setup you need to first loosen the stem, then tighten/loosen the star nut/compression plug, then tighten the stem bolts again, while making sure everything lines up. With a threadless setup you loosen a lockring, tighten/loosen the top nut, then tighten the lockring.

Servicing? It's done exactly the same way, all else being equal.

Sweat corrosion? Sweat's going to get into a headset regardless of whether it's threaded or not.

Stems are not easier to remove in a threadless system. In a threaded system you loosen one bolt, sometimes tap it lightly to disengage the quill, and remove. In a threadless system you loosen the stem bolts, then the star nut/compression plug. Plus you get the bonus of having to re-adjust your headset every time you remove your stem in a threadless system, which is not a problem in a threaded system.

I will concede that it's plausible that a threadless setup is stiffer, but only because it's not easy to compare apples to apples. Many of your other complaints are a symptom of stem design, not because of the type of headset. Also, many of the disadvantages you state in a threaded system are really only an issue for someone who's constantly installing and removing parts. The average rider wouldn't really need to mess with their setup all that often.