Question for Jehovah's Witnesses - What Happened to the Biblical Truths?

Jehovah's Witnesses are very proud of their heritage, as manifested in such publications as the "Proclaimers" book wherein the history of the Watch Tower Society is presented. The Governing Body also publishes articles in the Watchtower magazine that promote the idea that the Witnesses have been fed "spiritual food" of the highest quality, "food" that contains only Biblical "truths". For example, the study Edition of the Watchtower magazine (not distributed to the public via the door-to-door work), September 15th 2010, page 26, paragraph 4 states (available as a PDF download);

Christ found these same faithful anointed Christians doing their utmost to provide timely spiritual food for the household of faith. From 1879 onward, through thick and thin, they had been publishing Biblical truths about God’s Kingdom in the pages of this magazine.

So, the claim is that from 1879 onwards, the "faithful and discreet slave" (at that time believed to be Charles Russell, who composed his "Divine Plan of the Ages" by measuring the Great Pyramid of Giza), via Watch Tower Society, published Biblical truths in the Watchtower magazine (at that time known as "Zion's Watch Tower and Herald of Christ's Presence).

These "Biblical truths", here described as "timely spiritual food" included such teachings as;

• Christ had returned invisibly in 1874
• the recently revised teaching that the rapture would take place in 1878 
• Armageddon would start in 1914

However, were these teachings

1. Biblical
2. true

The answer, of course, is no. It's puzzling, then, that the Governing Body is choosing to describe them as "Biblical truths" and "timely spiritual food" when it should be obvious to even the most loyal-to-the-Watchtower Jehovah's Witness that they aren't. It's also curious that these "timely", "Biblical truths", as published by the Watch Tower Society, are no longer available; can you find these publications and Watchtower articles from the 1800s in your Kingdom Hall library? What about on the Watch Tower CD-Rom Library? Again, the answer is no; these teachings are no longer current*.

So, my question for Jehovah's Witnesses is this; if the food in 1879 was spiritual, timely and Biblical, why have the Governing Body so significantly changed the menu?

*Jehovah's Witnesses are trained to view such wholesale rejections of previous "timely" "Biblical truths" as the "light getting brighter". However, if the "timely" "Biblical truths" published by the Watch Tower Society during the late 1800s were of such a high quality that Jesus Christ appointed the corporation as his "faithful and discreet slave", why would they need to be abandoned at a later date? Didn't Jesus know that the "timely" "Biblical truths" published in the Watchtower magazine in the late 1800s were neither timely nor true?