I think Paradox did more bad than good allowing building supply hubs. In one of the topics about that, dev answered that at some point they considered removing option of building supply hub at all, but left it in game for the extreme corner cases - like late game stale fronts, important situational choke points and simply original hub placement design issues.
Building hub, from devs point of view, is rather a nuke-like option, and should be used on extremely rare occasions. But because it was added on the same UI layer as other, more usual, province buildings, most of players consider building a hub as a legit common way of dealing with army supply, and of course frustrated about 20k cost.
I was in the same boat and ranted about 20k supply hub after release, before I saw that it was not the idea to make player build it in the first place. Sadly, Paradox forgot once again that playerbase can not read dev minds, and it was very poorly communicated what hubs building is intended for.