Strategies for when your employees die

I had a similar issue and created a kind of difficult and annoying process to HELP it, but not SOLVE it. I started off with sims with low skills and built their skills up over time. I hired them at varying ages, which is KEY to this process. To find out ages, you can either get to know employees before you hire them by befriending those you know are available, or do it a much more complicated and cheaty way. Needless to say, I prefer befriending them! Once all or all but the oldest sim are leveled up, the first sim to become an elder is booted and a new employee is added.

In my experience, it's better to have one new employee than all new employees! It works for me and I maintained 5 stars with a little extra elbow grease. But, it may not work out for you, and it is a slightly more expensive route.