All SSD would be super-expensive, looking at the price Apple charges for the flash upgrade on MacBooks (where it's just from 512 GB to 1 TB, which costs $500). So, for a Time Capsule with 3 TB (which is already quite small nowadays, I think) that would be around $3000 only for the storage.yes, plz go with all ssd, smaller more reliable so more tiny design
Also, SSD does not make to much sense in a backup storage, because of the high cost of flash storage in general and it's mostly writing at low priority as a background process.