But I worry that by chasing backwards compatibility and not just saying "guys, you need to modernise your libraries to do X and Y, otherwise you will never be Multicore OCaml-friendly" the maintainers will invite a lot of legacy cruft -- which will now be subtly broken -- that they will either have to fix themselves or just concede that it's impossible.
Then again, I suppose they don't want the Python 2 vs. Python 3 split (or Perl 5 vs. Perl 6 / Raiku; EDIT: It's actually "Raku" as a commenter below clarified). But I fear it will happen anyway due to the reasons above.
I very quickly fell in love with OCaml but eventually settled at Rust for these, and a few other reasons. It's an otherwise excellent language and has one of the most underrated compilers on the planet. It's a shame that it's moving so slowly... :(
EDIT: As for safe parallelism, here's what one of the maintainers said soon ago: https://news.ycombinator.com/item?id=22735176
It looks like they prefer to do the lowest level possible so people can build their own abstractions on top. Completely fair though, not criticising.