Or... your company should be training potential replacements. This is what the US military and "white shoe" consulting companies do. While expensive, it guarantees that critically needed skilled staff are always available.

I recommend the article "Up or Out: Solving the IT Turnover Crisis" [0] which gives a reasonable argument for doing exactly that.

Notes:

0 - https://thedailywtf.com/articles/up-or-out-solving-the-it-tu...