The main event
Many years ago, when I was CEO of Chicago Area Runners Association, we owned and produced the Shamrock Shuffle, the largest 8k in the United States.
We always attracted a large number of new runners, for whom a five mile race was a challenge - especially at the beginning of the year. Some would complete the event and spend the rest of the day in a local tavern - still proudly wearing their bib numbers - enjoying the status of having completed a running event.
To attract even more of these folks, one year I suggested adding a 4k. it was an odd distance and more doable for newbies and infrequent runners. We advertised it in our preliminary flyers.
It wasn't until the printed entry form was ready to go to press (this was before web sites) that the race director gently suggested the 4k was a bad idea. He was right and I wish he had expressed his opinion immediately, before I grew attached to the idea of another race.
He said the 8k was the "main event" and we should do everything possible to strengthen it and attract more runners. Adding a 4k would be a distraction and would divert our energies away from the "main event."
If you have a 5k that is floundering, don't just add a 10k. You may end up with two troubled races. Build up the "main event" first.