4e made the point that it was supporting "epic" adventuring beyond level 20. However, as your example illustrates, they didn't do that - they simply stretched 20 levels out to 30 levels. If a balor in 1e was a level 15 threat, that is what it should have been (IMO) in 4e (I actually made them level 18 elite threat). The clearest example are the spells: Meteor swarm is a level 29 daily spell in 4e. When do you get 9th level spells (meteor swarm) in 1e again? They didn't add epic - they just stretched standard D&D out to 30 levels.
IMO, Balors should have be level 18 elites and demon lords would be mostly level 18-25 solos and princes level 26-30+ solos.
Well, I don't think 'stretching it out' was REALLY the initial intent. I think that WotC figured most games would be like older games, the players would slog their way up through the heroic levels, and then maybe struggle along if they were really dedicated and playing a long-running campaign until they got well into paragon, but that the epic levels would be almost like a 'whole other game'. Something that the bulk of players wouldn't get to. Most players don't kill gods and unique god-like named beings and such.
TBH, IME, AD&D was a lot like this too. There were a few times, mostly early on, when the players ventured into the high teens of levels, but most campaigns only slowly evolved up to touch the edges of 'epic' play. So, for instance, the highest level PC I ever ran outside of games I played before the age of 15, was a 14th level Human Magic User. Given the nature of AD&D we COULD take on demon lords, it was just a VERY high risk endeavor that required vast preparation. At those 'beyond name' levels we WERE epic, and the rules of the game didn't even really work that well at those levels either.
I think 4e's epic was at least intended to work like that. Our epic play, and I think what I have read of [MENTION=42582]pemerton[/MENTION]'s and others who played similar games, was that epic 4e was 'gonzo'. It wasn't really played 'by the book'. Sure, the book acted as a framework, but the rules were more a starting point than where epic lived. Ours was filled with crazy stuff, one off rituals, stupidly powerful consumables created with crazy stuff, etc. In that sense I do identify a pretty close comparison with earlier D&D, and I don't think levels 21-30 were just 'stretched' into existence.
That being said, I think MECHANICALLY they were redundant. I wrote my own 4e-like with 20 levels, 4 of which (17-20) act as a sort of epic tier.