>sure, but would we even be here talking about VS Code if it had been written in those languages? Would the extension community have grown as quickly?

Why not? Didn't hurt ST or TextMate much (though them being closed source did). And we still use Vim, written in C, 40+ years on, and it has tons of plugins...

>Would Microsoft have continued investment in the product if it hadn't?

MS developed Monaco for over a decade, with very little adoption, so why not?