|Pyro as Bill said:
They could still run the game from the HDD but if they wanted the fastest load times they'd need to copy/transfer to the SSD. Last gen games and smaller current gen games would all load at the same speed they do now from the HDD.
Online games (waiting for other players to load) and games with a lot of streaming (which are most big games) should really be on the SSD at all times. Last gen games and cross gen games can be on the HDD sure, however with a 250GB SSD, OS takes 100GB or more, you're already left to 2 games. I also doubt it would be $100 less at retail to go to a 250GB SSD. But we still have to see what the specs are for the Series S. Perhaps that will have a much smaller SSD. It should not have a HDD or hybrid drive though, at some point developers will need to have the freedom to leave tragically slow streaming behind.
You're right. The price difference is probably closer to $50. SSD prices have come down more than I realised and 480GB and 1TB seem to the best price/size option.
Still, the amount of space needed in the first year isn't the same as in later years so I'd still take the £50 price drop with a 240GB/480GB and add my own USB SSD/HDD as and when it's needed.
Can Sony and MS's proprietary SSDs be replaced with larger sizes or can you only add to them?