in most 'mainstream' games new content puts all players on a level playing field
So, are you suggesting that every time new endgame content is released that everyone be raised to the level of the then-current top guild?
You realize we aren't putting out an entirely new game every year like those billion dollar games, right? It's not reasonable for us to obsolete the entirety of existing content every year.
It wouldn't be too difficult to make every character turn level 65, get all of their AAs and 5 tomes done, and equip them with Tier 10 gear the second they log in, but then 90% of the content of the server would be obsolete and the complaints would be "there isn't enough content".
Mainstream MMORPGs nowadays are built on an exponential scale of power. Every year, the power is multiplied by X, so the power is proportional to X^Y, where Y is the number of years the game has been going. This is fine as long as the base power is only less by a factor of X and that a full game's worth of content is created every year to create an entirely new experience that exists separate of "last year's content".
SoD is built with the intent of avoiding this rate of growth while acknowledging that some growth must exist. The experience benefit is essentially logarithmic for this purpose. After getting level 65, the benefit from each experience point goes down and down and down. The first 100 AAs you get are amazing. The next 100 AAs are also great, but your hidden AAXP bonuses fade over this time. The next AAs slow down in benefit, and you start considering tomes.
The Codex of Power is then a 20% increase in power for an experience cost higher than your AAs, but the benefit is less. Past that, if a wizard wants a 4% increase in their nuke damage (equivalent to 1 codex of power, 110 AAs), they'll need to do Tome of Flame/Frost/Magic Mastery I/II. That's 6 Ikisith Tomes for a total exp cost of 280 AAs, about 2.5 volumes of the Codex of Power in total length.
What does this say? This says you can get to "competitive" for the top 5 guilds experience-wise in about 1/5 the time the established players have spent. That doesn't take into account that it's easier to get experience now than it was for the people that have been playing for years. Along the way, there's a rich amount of content that is thoroughly enjoyable that nobody wants to render obsolete (except for people that want to have end game characters and a brand new game to play and that it happen right now).
If you want to be in a high-end raiding guild, you can manage that in less than a year while experiencing a lot of content built over years by passionate developers. If all you want is to jump straight to the very top so you can "win the game", then you can do that too with sufficient effort. However, asking for everything that has been built over the last 8 years to be tossed away for a single moment of being the equal of those players who will claw and scratch their way over you to reclaim a spot at the very top (and they will succeed, they've already done it once, and I know this because I'm one of them) achieves nothing.