Not really that surprising.
The platform itself has yet to experience decline. They just had a record year where they sold over 20 Million units and they're in no real danger of losing significant marketshare at this point despite the Switch taking off in a really positive way.
The PS4 itself remains hugely profitable and a bunch of Sony's own 1st party developers are yet to push out their 2nd games. There's enough time for another revision, particularly on the PS4 Pro, and more than enough margin for them to give both the standard model and Pro a hefty pricecut to push it towards Wii, PlayStation and potentially even Gameboy numbers before it's phased out.
I don't expect to see PS5 until at least the holidays of 2019, but I wouldn't be surprised to see it be pushed to 2020 depending on how the market reacts to Switch and the PS4 strategy throughout late 2018 and early 2019.
