There's plenty of small business owners who are perfectly capable of managing the risk of losing staff for whatever reason without it sinking the company. Many business run at a big loss when starting up, never mind razor thin margins. The ones that survive manage their investors, debts, cashflow and staff risk properly.

If you run a business on the assumption that you're going to have all your current employees working for you continuously for the foreseeable future, you're going to have a bad time.

Parental leave shouldn't be unplanned though, there should be several months of notice surely