I've read/heard that argument before, and it's flawed.
All content is from level 1. Every quest, dungeon and other activity available to the player from the moment they step foot in the world 'til they've reached "the end" or have decided not to continue is the content. In that regard, end-game only accounts for about 10-20% of the overall content in WoW, or in any MMORPG.
People tend to confuse the content that they are personally interested in with what is "the real content" or "the only content worth doing". It doesn't cease to be content because someone isn't interested in it, or doesn't feel "it's worth doing". It's still content, it's still there to be done and it still counts. Because someone chooses to ignore it doesn't disqualify it.
The whole "end game is what matters" thing - which your remark seems to echo, zumi - is a 100% player-invented concept, derived because it A) is the equivalent of "winning the game" in a single player/console context and B) is where "the best loot is", which is all many people play for these days anyway. They're always focused on "getting the best rewards", and little else. They're 100% reward-driven.
This is why many MMO gamers turn the games into complete grind-fests for themselves, and then later come back to complain about it, as though the game forced them to play that way (many have made that claim across many MMOs) and provided no other options. Now, some might say "but wait, the game provided no other valid options" - I've seen that retort as well. My response to that, is to ask "valid by whose standards, and on what basis?" Oh right, based on "what gives the best rewards in the least amount of time", which brings me right back to them being focused entirely on getting to end-game and getting the best rewards.
If people would take a step back, realize they are victims not of "poor MMO design up to level cap", but of their own narrow-minded "gotta get the best stuff ASAP by grinding non-stop 'til I get there" mentalities, they'd start to realize that, "gee, most of the content actually is at the lower levels, I've just been racing past it all this time because it didn't have the best loot. If I slow the hell down, stop pushing myself to end-game/better loot every moment I'm logged in, and just freaking enjoy the journey more, suddenly there's all this content to do that I've always ignored in the past. Maybe playing MMOs doesn't have to be all about "maximum productive efficiency" as I've been telling myself all this time".
I don't actually expect that to happen, by the way.
That's why there are people who can play these games for years and never get bored. They aren't blazing through them to end-game, getting through that, getting bored, blaming the devs, and feeling the only thing they can do is move to a new game.
Do not confuse "the content that matters to me" with being "the only worthwhile content the game has", and that's right in line with "most of the content is at end game". It's a nonsense claim.