Offer to teach the developer something.
"Java developers - want to learn Node and React?"
"Rails developers - want to learn Elixir?"
Get one senior person on staff who really knows the stack well and require that they pair program every day with a different member of the team.
Good developers are grateful for the opportunity to level up their skills, contribute, and still make a living.
Andres Camacho in SF has been VP of Engineering and CTO at several startups and is king of this strategy.