I think that Esperanto would already have become popular as a second language throughout the world had the Esperantists themselves recognised one simple fact: Esperanto does have its opponents.
The Esperantists only want to hear the 'good news'. If you try to tell them what's going on behind the scenes they'll generally tell you to shut up. What I came across was systematic harassment of key people in the Esperanto movement in the UK, before I was subjected to the same thing myself. There is undermining going on, and it's not just the paranoids who have been trying to bring that to their attention.
Now it's spilling over into vandalism. See:
Vandalo atakis monumenton de Zamenhof — Libera Folio . During the World Esperanto Congress this summer there were at least seven incidents of vandalism directed against Esperanto. That wasn't just bored youths.
We got an Esperanto parliamentary group going in 1972, and after the 1974 general election we had a majority in the Commons. Shortly afterwards there were dirty tricks against the president of the group, who promptly resigned. There were dirty tricks against the president of the Universal Esperanto Association in London, who resigned in 1974. Harold Wilson, himself and Esperantist, resigned as Prime Minister in 1976 following dirty tricks. There were dirty tricks against the Esperanto Parliamentary Group in 1999, leading to its collapse.
The next lingua franca could still be Esperanto. The idea of Esperanto will make sense to people after the current 9/11 wars, in which English is being pushed as the language of a militaristic New World Order.
Esperantistoj, vekiĝu.