I think there's an argument to be made in favour of launching later - 2020 or so.
But a 2019 launch would probably give the PS5 a year on the market to itself, whilst MS can wait it out and launch at a higher spec.
The sales thread has shown us that the XBox division is profitable. So, if the X1X keeps selling well, and keeps people buying games in the MS ecosystem, there's evidence that a year long wait is fine, as long as it provides better hardware.
I don't expect the PS5's release to be effected by PS4 sales. As others on this board have said: the PS2 sold tens of millions after the release of the PS3 and X360. A PS4 Super Slim and a PS4Pro Slim would still be attractive prospects and could sell for many years into the life of the PS5, especially if the latter is backwards compatible.
The PS4 will become the cheapest entry point into the PlayStation ecosystem and routinely be sub-HD, the Pro will become the standard 1080p-1440p console. Developers will target the ~100 million PS4/Pro platform, and offer enhancements for the PS5. Sony will only charge a single licensing fee for releasing a game on the PlayStation ecosystem - maybe offering some kind of incentive for implementing PS5 enhancements.
-- Onto hardware --
I expect that the APU of the PS5 - in particular, its size - will be designed around 7nm+ which is due in 2020. In order to release in 2019, they'll utilise 7nm, manufacture a larger chip, and absorb the costs, knowing that 7nm+ will render it break even (or the PS4 style of break even: a game and year of PS+) a year later.
As an example, and pulling the figures out of my arse:
- the PS5 launches in late 2019, on 7nm, with a 450mm2 APU
- when 7nm+ is ready, they transition the APU to that node, for a 350mm2 APU
- there's no redesign of the chassis, no slim at this point, just a modest internal revision
On the topic of revision: I'm going to revise my RAM estimate to 16GB of GDDR6 or HBM3, with barely any of it dedicated to the OS. I'm sticking with the NVME prediction though.
DDR4 is fast enough for a 4K console but it's expensive. But it's also cheaper than GDDR6 or HBM3, so using a healthy chunk of it for the OS and apps will come with a tradeoff, which I think will be the "game memory."
This has me thinking though: GDDR6 can only reach its crazy bandwidth when there's 24GB of the stuff on a 384 bit bus. Would 16GB offer enough bandwidth? Would 16GB be a small enough amount that HBM3 becomes cost effective?