I don't think anyone realistically claims that it will. Not only did the PS2 have almost no competition early on (the Dreamcast posed little threat and was discontinued months before the GC and Xbox launched), plus it had absurdly strong legs, with a significant proportion of its sales coming after the start of the seventh generation.
Gaming was at it's peak in that era, now it's slowing down...a lot
Hm. 260 million seventh-gen systems vs. 210 million sixth-gen systems. Seems like the seventh was the peak. Of course, it's debateable as to whether the market is actually declining in terms of number of customers. I think a good argument can be made that much of the sales growth over the past several generations was due to atypical circumstances causing hardware sales to grow at a faster rate than the number of actual console owners. The sixth generation total was arguably inflated due to the PS2's record-setting legs, and the seventh was inflated by the Wii, which likely increased the number of multi-console homes considerably. This is a topic warranting more in-depth analysis, which would result in this thread going off-topic if continued here, but needless to say the fact that the eighth generation will almost certainly mark the first time total generation-over-generation hardware sales decline may not necessarily be symptomatic of an actual market contraction.