PS4 will end relatively close to PS3, possibly still ahead alligned and with better software sales and third party support. SW is not out of the woods either, considering it is massively behind 3DS in sales and has lost a lot of exclusive third party support (not sure where software sales are in JP compared to 3DS).
At the moment, its pretty safe to say PS5 will sell similar to PS4, which sold similar to PS3: 9.5-10 million, 60-70 million software. It will get all the third party support by default as Playstation continues to be where most 3P Japanese games are sold.
Problems could be high price, or weak software lineup, though from rumours it seems its going to have a better lineup than the PS4.