The problem is that Skyrim's biggest selling point is also it's greatest weakness. Because it is an open world in which you're suppose to be able to do quests at any level I guess they had to try and come up with a solution for the person who insist on doing the main quest at level 5 that would still work for a lvl 30+ character. I wouldn't be surprised if Alduin is actually bugged and were meant to level with the player in the same way that dragon priests do. Either way, as long as players have total freedom, boss fights will be tricky to get right. This is probably why enemies leveld with players in Oblivion and look how people complained about that.
Just to add to this conversation, if you do as I did on my second character and wait until level 35 or later to start the main quest it is a lot harder. My first random dragon was an Elder and it just got worse from there.