Well, I think you're getting caught up a bit too much on the minority. Obviously, hardcores are usually not fresh players. Then again, as a fairly hardcore player myself, I joined at Far Edge of Fate, and STILL haven't actually unlocked Final Coil or any of the Savage Coils because I refuse to unsync them and finding a party otherwise is hell. Active end-game raiders are probably a majority of long-time players, and a not insignificant number of newer players. Once you get into casual, which makes up a vast portion of the playerbase, then old content is not so "old" anymore.

The class changes do affect old content, but they it make the content too easy, you can make it harder yourself (MinIL, or MinIL undersized). If they make it "impossible" the game would be broken and SQEX would have to fix it. If they make it significantly harder, maybe you'll have to take some unconventional approaches to clearing it. But most stuff can be done in strange, unintended ways in this game. For example you can 4-man Ultima's Bane with no tank on Synced. If they let us customize the sync level, it could be done at a significantly lower item level as well.