It is really quite simple to assess if someone is 'in a rut', even more so when they have more experience, as it becomes more obvious. Even with mid-career developers I check for this.

So "My only concern hiring older engineers is that they are stuck in a rut." doesn't come across well, anyone reading this should be very clear that to adopt that thinking when interviewing would be very lazy on your part!

Not trying to bash you personally, and I suspect you just didn't express what you meant so well :)