1) The composition of comets were largely unknown until ~1950 when Fred Whipple hypothesized that they were largely icy objects instead of rocky objects, which was only confirmed in 1986 when Vega 1 and 2 flew by Halley's comet.

2) This idea was flipped in 2001 when Deep Space 1 took photos of the comet Borrelly, which showed a hot and dark surface.

3) The Stardust mission in 2004 discovered that the dust of a comet was actually a crystalline structure, created by very high temperatures.

4) The Deep Impact probe in 2005 was the first physical contact humans had with a comet's surface, which resulted in us combining the two previous hypotheses into an object filled with an icy dust mixture protected by a hard and hot dirt shell.

5) Finally this year we landed Philae onto a comet's surface by launching a probe in 2004, a mission that started as far back as 1992.

This means that we were planning on landing an object onto the surface of a comet long before we knew what in the hell a comet really was. To say that we failed to meet the goal of a one week operational timeframe on the surface disregards the entirety of knowledge learned during the operation of the mission in which we were conducting. Just after Rosetta launched we figured out that the surface of a comet was probably more ice than rock, and had to conduct retroactive experiments in order to determine whether or not the harpoons of which had already been engineered, designed, manufactured and launched into space (which failed to work, so the result is somewhat moot anyways) would in fact be capable of holding the lander on the surface of the comet. The main goal of this mission was to learn more about comets, not spend a defined time on the surface like a game of "King of the Mountain" at some elementary school playground. The mission was successful. We obtained data about the surface of comets and discoveries and observations resulting from the collection of this data will continue to happen over many years to come. Would have it been better if we were able to keep a probe operational on the surface of the comet for years instead of weeks, or weeks instead of days? Yes. But the goal was to learn shit, not get in a pissing contest with a nonexistent alternative lifeform that has already landed an object on a comet and kept it alive longer than us.