So, as I said, it's not something done overnight. It's not normal (you cite one example of a low tech track ride where track accuracy is not all that important, and the original track was of mediocre quality. I've seen pics recently of a coaster that received a few feet of replacement. Can't remember which one, but it wasn't an overnight secret job.
And a perfect weld doesn't just mean a code stamp quality weld, but a weld that puts the new track in the exact location in space. You can grind all day, but if the new track was rotated a bit when welded in place, it will create a new problem.
And there ain't no way Universal was creating new track pieces for a B&M invert, unless it was the brake run or station. Modern track is bent by INSANELY expensive machines. Not a portable pipe bender and a couple of maintenance guys.