Going by history, backwards compatibility is not a primary prerogative of Sony's. And going by current trends of PS4 sales, why would Sony want to butcher PS4 hardware sales by allowing backwards compatibility with the PS5?

PS5 is not going to launch in a non-holiday month. Mark my words. 

EVEN fall 2018 is more likier than spring 2019.

The ignoratio elenchi fallacy is really getting annoying on these boards ... (How about you guys actually start showing how the opposition's argument will further yours ? That shouldn't be too hard from now on, right ?) 

What you say about PS2 doing "amazing" numbers isn't true either considering the system peaked in it's second year. PS3 replaced PS2 earlier because the latter was reaching a plateau in it's main market and had hardly any further room for growth ... (just before the launch of the PS3, PS2 was selling for $129) 

double post

The reason I think mid 2019 is to get the jump on MS. MS released their mid gen console a year after Sony so it's less likely Sony will get a huge backlash from those who only just recently bought a Pro. MS would have to wait a little longer in order to make sure they didn't have a backlash. Pro wasn't promoted heavily whilst the XB1X was, which makes it even harder for MS to try and release alongside Sony if Sony decide to release mid 2019. This would give Sony at least 6 months head start in building a userbase for the PS5, unless MS take that big risk. Throw in BC and as far as I'm concerned it's game over. 

I don’t think that Sony ever expected the Pro to take off. They always marketed it as a nice bonus for PSVR and 4K TV owners, but never as an essential upgrade like MS did with the X1X. They for sure know that price is the big deciding factor, not power.

The last gens the architecture changed heavily from generation to generation. Next gen will most likely also see an X86 based system, which imho makes BC very likely. 

Sony can easily miss out on hardware sales for PS4 if it benefits the PS5. The sooner they start building their future the better. It could even benefit PS4 sales late in the cycle, if people would know all their games would also work on the next new system.

I can’t imagine they’d leave the field of BC to MS, when the main reason for no BC on PS4, the Cell’s difficult architecture, is not a thing anymore.

Probably 2020, cause that should give Sony enough time to develop decent launch titles

2020 or 2021.

Why would Sony kill off the base PS4 when it is perfectly capable of offering PS4/XBO games at 720p -1080p for years to come? They can get into homes for $100 in the next year or two, while adding HDMI 2.1 and a 4K Video chip to make it one of the best streamers on the market. It is an excellent option to combat the Apple TV, Amazon Fire TV, Chromecast, and Roku on the low end set top box market.