You don't need to play the long game cost-wise regarding storage, it's not a CPU or GPU which needs to stay more or less the same the whole generation. You will have model refreshes and different storage solutions over the generation, just like any other generation. If starting the generation with a 256GB SSD + 2TB HDD makes sense cost-wise, if 3 years later a 2TB SSD will make more sense than the "S" or "slim" will have a 2TB SSD instead. As long as you upgrade over the years instead of downgrade, it will be fine. Right now 2TB SSD is way too expensive, 4 times more expensive than the 500GB HDD the PS4 and XBO had in 2013 so 1TB is probably the only option for a full SSD storage solution. Right now the retail cost of a 1TB SSD VS a 256GB SSD + 2TB HDD is the same. Will it stay cost the same in 18 months? I don't know, but right now going full SSD with just 1TB storage, that's pretty small and limiting for the end user, it might even create a media backlash.
Right now the X has only 780GB available for games and apps out of its' 1TB drive. Games that are X Enhanced are ~100GB so a lot of next-gen games will probably be bigger than that. We are pretty sure that Halo Infinite and Forza 8 will be launch titles, installing Forza 7 and Halo 5 will take up to 200GB of your X HDD and that is before DLC, so I'm pretty confident that Forza 8 and Halo Infinite will take more than ~1/3 of your HDD on launch day. Do you remember all the stories about "XBO-X HDD is too small"?
Like this one,
this one or
this one? If next-gen launches with 1TB SSD it will be even worse.
Cache drive is the most logical option. Microsoft is already halfway there with their DL project
Fast Start which uses DL to prioritize game blocks for early download in order for games to launch faster while downloading. That kind of project is perfectly suited for a DL system that smartly caches partial game data on the SSD from your main 2TB HDD or external drive. It also allows for the use of external drives and cheap HDD upgrades.
And just one remark regarding SSD price drops, we've already seen the lion's share of their price drop. The biggest factor in that trend was oversupply so don't expect SSD prices to drop much lower than 10c per GB, a price they are pretty close to right now.