When I got LR5 first, I was new to editing and my enthusiasm drove me to get down to learning how to edit. I wasn't interested in the filing bit too much. Now someone has told me my output should be separate from my originals as I could lose everything. No1 moving anything would be a huge chore and No2 I wouldn't know how to. I have had a couple of occasions where I disconnected drives and spent tedious hours trying to re-connect "disappeared" files or folders.
There is absolutely no problem keeping everything on the same disk.
You do however have to make regular and automated backups of your Lightroom catalog and photos TO A DIFFERENT DISK. This is where the risk comes in, if you don't have this, you are accepting extreme risk that you will lose everything. If you do have this, the risk of losing everything becomes extremely low (fire, flood, meteor strike). And if you also use a "cloud" backup service, then your risk is nearly zero.