I can't really blame developers for this -- they're just responding to incentives. So long as hiring managers penalize candidates that don't have experience with trendy stacks then your actively doing your employees a disservice by prohibiting them. The only thing I can perosnally do to combat this problem is be conscious of it, not engage in that kind of hiring behavior in my office, and hope that the culture changes.

Giving employees opportunities for side projects helps somewhat. Allowing for gradual migrations to new technologies helps as well.