The only way I see PS getting away with holding off to announce PS5 until 2020, is if they pack 2019 with first party AAA exclusives. That alone should be enough with all the third party and indy games in between, to keep people happy and busy enough that they don't really notice the lack of events. PS4 price drops would also help.
Otherwise if next year isn't that much different than this year in terms of first party exclusives, then I would assume the PSX event will be a huge 25th anniversary celebration with the full PS5 reveal. Whether it launched soon after or in early 2020, I don't think it would matter too much if PS4 is locked at $199 by then.
Thats the thing though. By the end of this year the PS4 should firmly over 90M consoles sold to consumers. What possible point do they have to prove anymore as far as the PS4 is concerned?
I also expect that the new PS4 price is going to be locked to $199 for all of 2019. They can just sit back and let it trickle 10M worth of sales for all of 2019 and it will probably do betier than that anyways.
They put everything off and announce a PS5 in december 2019 to launch in march 2020 and all will be forgiven.