why only one end to the game?

The game starts with different starting zones, based on your choice of race. Each is different and interesting, some are better than others, but eventually everyone converges into the same few zones, such as the Stranglethorn Hole. After that the game then progresses with some divergence and paths, but all eventually leading back to the same end-game series of instances, and eventually to Black Temple.

The design concept underlying all this is that all roads lead to Rome - a plethora of choices early on, narrowing as you go, and ultimately being funneled towards one end destination. This idea is all well and fine at first blush, but there are problems becoming very evident.

Problems like: the only way to add new content is by shifting the goals further down the road, resulting in an even longer leveling death-march; and then whatever was the previous end destination becomes obsoleted and henceforth completely skipped; end-game epic gear becoming obsoleted by the trash greens that drop in the expansion; and fewer and fewer choices of where to play just at the time when your character finally has lots of capabilities (spells, talents, skills, etc).

What if the game had multiple ending zones, each rivaling the other, each requiring the same massive investment of time and effort to gather necessary faction reputations, resistance gear, etc. Now, not only could you experience a totally different game by choosing a different starting zone, but you could experience a totally different game by choosing a different ending zone.

Importantly, pursuit of one particular ending zone should come at the opportunity cost of the other ending zones - the simplest means might be via different resistance sets, more complex means might involve rivalrous factions. The point being that you don't get everyone congregating in one end city/zone/instance, but instead are spread out over different places. It should still be possible to take your champion from one place and go explore the alternative end zones, but the success in that first place shouldn't result in easy success in the second place (due to gear etc). Some advantage, but not much .. the same way success in the PvP arenas doesn't result in being equally suited for success in PvE raiding (and vice versa).

This divergent-endings design could be further enhanced by introducing meaningful faction rivalries, such that it might be quicker to level an alt to take on different ending, rather than grind down the hatred your main accumulated with the faction in charge of the other end-game instance.

I can imagine entire guilds being formed around different end-games. Fellowship of the Mountain, Black Rock Raiders, Underdeep Underdogs, Southsea Buccaneers.

Each expansion of the game could introduce new end-zones and instances, but no raising of the level cap (by not raising the level cap, you don't further exacerbate the death-march grind required to reach the level cap). Introduce new talents and gear and potions and enchants which are tuned for that expansion, but all at the same level as what is already in the game.

This idea isn't without it's own problems of course: many players get precious about their one character, and after the hard struggle to topple Illidan they would want to take the same character into the content of the next expansion. Counter that by ensuring there is enough divergent paths through leveling content, with the barest crossovers and melting pots like Stranglethorn.