I would assume most engineers would like to spend time building stuff instead of migrating already built stuff to a different but kinda similar language.
I don't think it's unreasonable to not want to do a CoffeeScript to TypeScript migration full time for an entire year. It's basically the equivalent of creating a "tech-debt" team and giving them only tasks which pay down tech debt. Sure, there are situations where tech debt needs to be paid down and you could make an entire team do it for an year, but the engineers will probably not _like_ being on the team.
I don't think it's unreasonable to not want to do a CoffeeScript to TypeScript migration full time for an entire year. It's basically the equivalent of creating a "tech-debt" team and giving them only tasks which pay down tech debt. Sure, there are situations where tech debt needs to be paid down and you could make an entire team do it for an year, but the engineers will probably not _like_ being on the team.