How's the PS4/XB1 vs PS3/360 graph looking now? I'm guessing it's starting to turn in favour of last generation considering we're at the point where PS3 was recovering and the 360 was picking up steam (although not at the point of Kinect yet).
If we're only looking at those two, it's not even close to turning in favor of last gen. Since Microsoft launched a year earlier last time, but they came out in the same month this time, there's two ways we could compare:
1. Launch-aligned, so Nov 2005-Jan 2009 for 360, and Nov 2006-Jan 2010 for PS3
2. First three years with both on market, Nov 2006-Jan 2010 for both
But why bother choosing? Let's add option 3, cumulative sales of both machines from Nov 2005-Jan 2010: the first
four years of 360 and the first three years of PS3. That total is 30.4m. By contrast, the combined sales for the Nov 2013-Jan 2017 of Xbox One/PS4 are 33.8m. So even with adding an extra year of sales to last gen, current gen still has a lead of ~3.4m units.
That gap will shrink in the next year, but not disappear. Last gen sold 11.1m units in that period, and I'd project current gen to hit ~9.8m. In 2011/2018, the numbers should be something like 11.8m/9.1m. So last gen takes over in January 2019...except all this is option 3, including that extra year of 360. Remove that, and you get option 2: last gen's HD twins don't pass this gen's until the end of 2019. Make the comparison launch-aligned as in option 1, and it'd be near the end of 2020 before last gen passes this.
Your guess is years too early. Whether almost two, three, or four years too early depends how you compare.
Total hardware sales will go down, yes. But that's mostly not due to Sony and Microsoft, but Nintendo. (And even that would be mitigated if you count WiiU and Switch as both being this generation.)