I feel like xcloud is going to be a way for Microsoft to get around the storage issues that come with switching to an ssd. SSD’s are crazy expensive and I very much doubt game sizes are going to go down, most likely they will almost double. So if Microsoft sticks a 1tb ssd in their console, it will hold what, 5-8 games? Juggling games on that thing is going to be super annoying and buying bigger storage options is going to be cost prohibitive for most people. I think most people will opt to just stream it instead of downloading it and having to constantly manage their storage space (Assuming streaming owned games is a feature of Xbox live/game pass and not some additional fee).
Microsoft isn't switching to an SSD. Their new console will have an HDD as usual. What the SSD is about is support for the RAM, because an SSD is cheaper than RAM. The SSD will be used to temporarily hold data for the game you are playing in order to reduce loading times, so the SSD capacity should be in the range of 32-64 GB at most. Perhaps more if Microsoft expects games to need more toward the end of the generation.