As someone who's watched the rise of R and Python from its beginnings, who pulled my hair out in frustration over the longevity of MATLAB long ago, and who has tried just about every numerical programming language it seems, I think the threat to Julia probably comes more from other LLVM-based languages like Nim or Crystal, or things like Rust or Kotlin.

Julia's adoption rate is really impressive compared to R or things like numpy, etc. so I'm not worried about that. But I do think it will have to contend with a number of competitors in the same space.