I don't really see map with diff spike intro. Only the whole map is a diff spike https://osu.ppy.sh/s/499478. Also to answer your question : ranked map have certain standards that has to be met to ensure quality. Like you said, practically all song have intense part at the end because the song has to build up the intensity. So it makes no sense to have diff spike early and such map could not be ranked UNLESS the music actually reflects it.