The PS4 isn't going to die fast.  The only PS system that died quickly after its successor launched was the PS3, and there's 2 good reasons why.  First, because of its custom HW, Sony couldn't cut the price quick enough to keep interest in the system.  The PS2 was able to drop to $99 shortly after the PS3 launched.  I think it even got to $79 at some point.  That was cheap enough for people to pick up an extra one. Or poorer families to invest in one.  The lowest the PS3 got was $199, with some stores dropping to $149 during its last year to clear out stock.  The PS4 won't have that problem, as the components are mostly off the shelf.  It'll be $149 before the PS5 comes out.

And second, last gen lasted 8 years, if you start at the 360's launch. People were losing interest in $199-$249 7-8 year old HW and were begging for next gen. I don't think Sony is going to make that mistake, again. They called Pro a mid-gen upgrade and think they mean that exactly. I think their schedule will be 3 years til their Pro model, then 3 years til the successor. This would put the PS5 in 2019. It'll be a good time to launch, because early adopters will be ready for a new system that's 6x-8x the power of the PS4, while late adopters will be happy with a $149 PS4 or $249 Pro, since most games for the first year or two will be cross-gen.

So, basically, it'll be able to be cheaper than the PS3 and Sony won't stretch out this gen til the PS4 has worn out its welcome. I could see it doing 20M+ in the 2-3 years after the PS5 launches.

I see it a little different. I see the reason for long lasting sales pf PS2 in two facts. One, the global distribution chains werent as well established as they are now. In some regions the PS2 just had started sellign a year or two before, and Ps3 wasn't on market yet. The second reason is, that the PS3 started slow. As PS3 seemed initially unattractive many games still came to PS2 and many people still bought the device. If Ps5 starts well, game development will switch fast and PS4 will fall down in sales in a year or so.

Then, can you explain away the decent legs the PS1 saw after the PS2 launched?  Sure, they weren't as good as PS2's, but it still sold 23M from 2001-2004, after the PS2 was launched WW.  I think the PS4 is going to do something similar.

I meant to reply to this last week, but I started having internet issues when I tried doing so and never got back around to it.

In any case, I honestly don't think it's a given that the PS4 will pass the Wii, and if it does it'll be damn close. Earlier in the gen I figured 120M would be an easy target for the PS4, but now I'm not so sure, and I think it'll sell a bit less than that, with 110M being as high as I'm willing to go.

The PS4 is trailing the Wii big time in the U.S.:

With a deficit of 10 million units and growing the PS4 has a lot of catching up to do. That's not going to happen this year. The Wii sold just over 7M in the U.S. in 2009, whereas the PS4's best was 5.74M in 2015. Now, the Wii did peak early and decline fast, so we can expect Year 5 to be the time when the PS4 starts to narrow the gap. The Wii did only a bit over 4.5M in 2011 and just shy of 2M in 2012, and about 1.1M combined between 2013 & 2014. But the PS4 will have to sell at least another 23M to pass the Wii in the U.S., whereas so far it's sold just shy of 19M. I don't see that happening. While it will narrow the current gap, it won't close it. In fact, it won't come even close to doing so. Personally, I think the PS4 will at most sell 35M in the U.S. lifetime, giving it a final deficit of about 7 million units.

As for Japan, the Wii sold 12.73M there, while the PS4 will likely sell only about as well as the PS3. That means the PS4 ought to have a final lifetime deficit of at least 2 million in Japan. So, between the U.S. and Japan combined, the PS4 will likely fall at least 9M short of the Wii.

That leaves Europe and the rest of the world to make up the difference.

According to VGC, the PS4 was at 23M in Europe at the end of April. By comparison, they had the Wii at 22.5M at the end of April 2010. That means the PS4 is holding its own against the Wii in Europe just fine. Considering the Wii's sales in 2010 and beyond, the PS4 will continue to maintain and grow its lead over the Wii. VGC has the Wii's final tally in Europe as 33.75M units. The PS4 ought to be coming close to passing that mark by the end of next year. The PS4 could easily reach 45M units lifetime in Europe, which would make up for the deficit in the U.S. and Japan. If the PS4 can do at least as well as the PS3 in minor markets outside the NA/Europe/Japan trinity, the PS4 ought to end up at around 105M, just under 4M units ahead of the Wii.

So, I don't expect the PS4 to pass the Wii at least until after the PS5 is announced. And of course, these estimates are contingent on when said announcement is. I'm assuming a Nov. 2020 release date for the PS5 in these projections. If the PS5 isn't out until 2021 or 2022, the PS4's legs will be a bit better, perhaps pushing it above 110M. But for now I'm assuming the PS4 will just barely squeak past the Wii before the end of its life cycle. And I honestly do not think the PS4 will pass the Wii before the end of this decade.