I used to have 4 cats. One of my cats loved to walk out and play around so much. One day, after he ate dinner, he walked out, but never came back. I don't want think something bad happened to him, I wish someone can have him and gave him a good life.
If you don't want your cats lonely, just adopt another one. They will fight each other at begining, but they will be fine later and never be lonely again.