I'm a big fan of SML as well (I worked on a hobby compiler for SML, and I've written some programs in it).

Investing in the ecosystem (tooling, VSCode support, packages) would vastly improve it, but it still has some warts compared to Haskell - no ability to auto derive implementations (like debugging/printing structures!), and lack of typeclasses can be painful at times.

I really like Rust because of how similar it is to what I believe SML could be - just take away ownership and change the syntax, and bam!