It's not ending soon, Kizaru was just confirmed to be arriving and Kuma is also arriving. Also as I said before we still have to see two backstories in this arc as well, Oda could also add other stuff during this arc to drag it a bit more longer.
30 to 40 chapters is not that long, if the arc lasts more than 50 chapters then yeah that seems way too long.
Some people also thought that the WCI arc was going to be a short arc with simply saving Sanji and escaping, but as we know that wasn't the case.