Are there a lot of people who actually want to use Elm tho? Seems like the real risk to me, not finding anyone interested in learning and using Elm.
Problem is that also attracts those where new technologies is not a mean but the end goal. So when they start to get productive in the "new technology" (that's now "one year old, ugh") and if the shop ain't up to the task of rewriting everything every year those people move on to the next hype baby . Being a geek-magnet should be a very minor and explicitly stated short-term factor in deciding what tools and technologies to use.
 Yes, this is anecdotal and based on one former colleague!
Elm is by far the best experience that I've had with any language.