I actually prefer that Irons and Dench are somewhat "anonymous" narrators, in that they don't receive an actual introduction by name the way Cronkite did.
I think the real star of Spaceship Earth should be the show itself, and not have a "star" narrator.
While there were only minor changes to the actual script when switching from Cronkite to Irons, there was a big difference in delivery, which contributed to the dramatic change in tone of the whole attraction.
The 1994 Spaceship Earth was much more mysterious, much more of an adventure you were participating in, whereas the Cronkite version comes off more like a live action version of a science class flimstrip. It's a more passive point of view and experience.
The 2007 version seems to be trying to blend the "on an adventure" feel of the Irons version with the "watching from a classroom" feel of the Cronkite version, and because of that I think the overall tone isn't quite as successful as the Irons version because I liked that sense of mystery. Now we're back to getting a history lesson.