Thanks to backwards compatibility, many of us want to move quite large software libraries from PS4 or Xbox One to their successors. New AAA games on the new devices probably won't be small either, so the internal 825 - 1000 GB will be filled up quite fast.
Both PS5 and XSX can be extended with fast SSDs, but neither option (PS5-compatible NVME-SSDs with strict speed requirements or proprietary Xbox-SSD-"cartridges") will be cheap.
Instead of deleting games from the SSD(s) and redownload them (or their huge patches) later, you could move them to an external HDD and move them back to SSD when you need them again. That should be much faster than redownloading them or reinstalling them from Blu-ray (plus redownloading mandatory patches). In some countries that would also ease the internet data cap problem.
Additionally PS4 and Xbox One games can be started from external HDDs and even if the loading times will be much longer than from SSD, I expect them to be noticable shorter than on the current consoles, since the "Jaguar handbrake" gets released.
So how much (and what kind of) additional storage will you probably add to your PS5 or XSX in the first year?