From time to time, posters (and indeed some 'reputable' analysts) predict that the PS3 will experience a large increase in sales to the point that it is believed the console will shift 15-20 million consoles a year for the next few years.

Equally, the opposite is often predicted for the Xbox 360, with people believing the console will effectively disappear off the map after 2008.  

I personally do not believe either console is going anywhere anytime soon.  I think the PS3 will see a marginal increase in sales over the coming two years, and the 360 a marginal decline over the coming two years.  I think they will each hold staedy levels for one year after that, before beginning a steep decline in the face of new consoles.

What makes people believe either of the first two scenarios?

The PS3 has some guarenteed sales in FFXIII and GT5, but the likelihood these games will provide PS2-like boosts is small, given both Japan and Europe are predominately one-console continents.  The Wii has already shown it will be the nearest thing to the PS2 of this generation.

As for the Xbox 360?  The reason that the Xbox died is Microsoft stopped supporting it and the games dried up.  But as of now, we have a year-on-year increase in sales, a strong line-up of diverse exclusives and every indication the games will keep coming for at least two years (ME2, Bioshock 2, Halo 4, PD2 etc...).

Does anyone have any idea why either of these predictions is made, and what reasonable grounds there may be to make them? 

Xbox360 is outsold by the more expensive PS3 despite having a better media support, being in the 2nd position, having better games.

Time is playing agaisnt the Xbox360 :

Each day the PS3 is getting more strenght. Games are coming (not all games are delayed and not all games are Lair-like lol), BR player becomes more mainstream (US numbers).
Xbox360 should have killed PS3 when it was very weak (just before 40gb) by slashing the price.
Now it is a bit late and The Empire is striking back.

Because the most of the people in vgchartz are Sony fans. Dont get me wrong...

edit: I am not saying that the PS3 will not see a big boost over the coming years. But my opinion is that XBOX 360 will never face the situation of the original XBOX. I am preaty sure that will overpass 55 millions in total sales!

^^ well said

Just remember what happened to the "Empire" in the end though

I just think that any predictions of a massive rise in PS3 sales or massive fall in 360 sales ignore a few things.

PS3 rise:

-Wii's success.

-Price, relative to other consoles (price was a big advantage for PS2)

-Late launch.

360 fall:

-Year over year rise in sales.

-Infinitely better game support from Microsoft than the Xbox, looking to continue for AT LEAST two years (and that's not counting unannounced games, and the fact that any games more than two years away wouldn't have been mentioned to the public yet). 

I don't see any Empire Striking Back. What games are coming? FFXIII for a month boost in Japan and MGS4 for a month boost in the rest of the world. BR becomes mainstream and so will the Blu Ray Players.
The Empire has fallen since it cant achieve half the numbers of its predecessor. Wii is the new Emperor (its is really funny when they say that wii is dominating because its for casual gamers and at the same they praise PS3 for its multimedia capabilities-non gaming thing), the 360 has achieved much more than xbox1 and PS3 laments over the ruins of its fallen stronghold. Forums and threads, don't make a console success.
Stracraft, good writing again!

One word: PStosterone.

It bugs me the most because everyone was using this same argument that 360 has no more line-up last year for 2008. It will happen next year again on VGChartz. After 2009, no more 360 line-up!

IT already is.  Check out this post from the Killzone 2 delayed thread.

krik said:

My expectations for the game just doubled. I was loosing interest but now that it's coming out in 2009 I'm excited again. 

same here my friend. i expect this to be huge now

along with god of war 3, final fantasy 13, final fantasy versus, gran turismo 5 and now killzone all set for 2009 make 2009 another tiptop year for gaming on the playstation 3.

killzone 2 will be a huge game!

I Have to agree, I see no sign that the 360 is loosing anything. I have both 360 and PS3. All my mates have a 360 and very few have PS3. But I do have to say the everyone I know has a wii and for the PS3 to get to a crushing level they would need to get to the wii levels and I really don't see that happening because most of the wii owners are former ps2 owners. I also have yo admit that I never use the PS3 apart from for media play as I thought I have to use it for something as it was a present and I can't sell it on.

Also where are all these AAA games for the PS3 I have been waiting now for years for a good one. Everyone keeps saying wait for MGS or KZ2 but every time I here anything about them they just say it has been pushed back a year, it really is poor...

And just to finnish off, I have GTA4 for 360 and last night I went round to my one mate how has it for the PS3 and let me tell you all this about PS3 better is total rubish, its blurred, frame rate problems and yellow'y for some reason, and he had a Pioneer 50ish plasam too which is bloody great. So don't listen to all the websites as they have a hiden motive, the real true is its better on 360 in about all ways, and to me that is what I expected as I am a bit of a techky person and have looked at the two systems closely and know what I am looking for when it comes to games.

Also is there any other independent source for sales out side UK in europe to check the sales figures with...