I don't think a portable is on the cards. Note I said 'think' which means I'm not 100% certain.
The future for me is quite straight forward. Once the PS5 releases (with full BC), the games on the PS5, with all the extra bells and whistles, will run on the Pro at lower resolutions, maybe checkerboared in some cases and even 1080p in others. The PS4 will also run these games but likely in 900p because of having to deal with all those extra GPU intensive bells and whistles.
I'm not even sure there will be ANY PS5 exclusives, at least for two or three years. Sony will measure this against the continued success of the PS4 and Pro. If the market continues strongly for these two consoles then they'll keep full BC for longer, if they slacken, full BC will end sooner rather than later.
There's also a good chance that the Pro will become the defacto 'last gen' console (in relation to the PS5), BUT even though that may shorten BC a little for the PS4, it won't necessarily shorten it for the Pro. It all depends on how the Pro performs as the main 'last gen' console. It could be too much of a risk to potentially annoy the biggest Playstation market at that point, which of course is the PS4.
The PS5 Exists.
