Not sure who told you this. It's not easy to replace anyone and hasn't been for a while now. An experienced individual leaving a team is always a big hit to the team's productivity. Typically takes 3-6 months to recover from that.
Ok, maybe not easy to do, but easy to justify. Why not get rid of an expensive senior dev and replace them with someone fresh out of school? They typically can work more hours (no kids) and can do 80% of the work for 50% of the pay.