I will probably buy a 1 TB expansion card for my Xbox Series X at some point, once the price on them goes down some. The 1 TB drive it comes with is not going to hold very many games sadly, current gen AAA games are already averaging over 40 GB, with some games like RDR2 and AC Odyssey hitting as high as 150 GB with DLC. If current gen games are already that big, next-gen games will be even bigger thanks to higher quality textures and other higher quality assets. 100 GB+ will likely become common for AAA games next gen, which means you'll only be able to fit maybe 8-9 average sized AAA games on the XSX drive, since some space will be taken up by the OS.
PS5 has an even bigger problem, because it is only 825 GB, with some of that taken up by the OS you may only be able to fit like 6 or 7 average sized next-gen AAA's on it.