Those who "focused in some super-niche proprietary tech for many years and are unfamiliar with the latest trends" have a level of talent that may be difficult t

Those who "focused in some super-niche proprietary tech for many years and are unfamiliar with the latest trends" have a level of talent that may be difficult to measure. It probably isn't low though, since they lasted years. Nearly nobody will hire them, because employers like you are expecting 100% domain knowledge on day 1. Because you are completely unwilling to give a person a chance to learn on the job, you are missing out on employees that tend to be low-cost and loyal. There is a cost to having employees regularly quit, and you are paying it because you refuse the sort who stick around.