I'm not saying mid gen upgrades are bad, In fact I've always been for it personally.But:
PS4 -> PS5 you will see/notice a bigger difference than
PS4 -> 4Pro -> PS5
Sure the flops between PS4 and PS5 may be the same, but it's then an incremental change. You will not see the huge difference because it happened incrementally.
Early days, but are people saying wow 4pro graphics are a huge difference?
That doesn't mean it's not a worthy upgrade though.
It would really depend on release dates.
Ps4 (2013) to PS4 Pro (2016) is a tiny jump and if they do a Ps5 in 2019 it would be a small jump from the pro surely.
But Xbox one (2013) to Scorpio (2017) will be a bigger jump and if MS waits 4 years again you have 2021 for the xbox next. We could see some very large gains then.
Of course Sony could wait also ps4 pro in 2016 to a ps5 in 2021 will be a big jump too or even 2022. I'd wager it would have to do with process nodes now and no really anything else.
Also since VR seems to be a hit for sony changes for rendering with that like eye tracking could lead to jumps that seem bigger than they actually are