> said staff was hired to work on the stack you are using
Looking back at doing various hiring decisions at various levels of organizations, this is probably the single biggest mistake I've done multiple times, hiring specific people using specific technology because we were specifically using that.
You'll end up with a team unwilling to change, because "you hired me for this, even if it's best for the business with something else, this is what I do".
Once I and the organizations shifted our mindset to hiring people who are more flexible, even if they have expertise in one or two specific technologies, they won't put their head in the sand whenever changes come up, and everything became a lot easier.
Exactly. If someone has "Cloud Engineer" in the headline of their resume instead of "Devops Engineer" it's already warning and worth probing. If someone has "AWS|VMWare Engineer" in their bio, it's a giant red flag to me. Sometimes it's people just being aware where they'll find demand, but often it's indicative of someone who will push their pet stack - and it doesn't matter if it's VMWare on-prem or AWS (both purely as examples; it doesn't matter which specific tech it is), it's equally bad if they identify with a specific stack irrespective of what the stack is.
I'll also tend to look closely at whether people have "gotten stuck" specialising in a single stack. It won't make me turn them down, but it will make me ask extra questions to determine how open they are to alternatives when suitable.