Hdds are workhorses. Old and reliable. Apart from complete failure, it's almost always able to access that data, even if sectors have gone bad. This is especially true if the OS is on a different drive. Easy to copy, hard to break.
SSDs on the other hand introduce some risk. Pain in the butt to copy do different size drives, and while they don't suffer from bad sectors if they do break, that's it, dead, no recovery possible.
So there's some element of risk on both drives, it won't hurt to not have a hdd for storage, until the day it does.
Personally, I'd keep the hdd, takes but a second to plug it in, boot, backup, shutdown, unplug power once a month or so. This way it's there if you need it, but don't have to listen to it on a...