"Typst Overleaf" sounds like a fine business idea to me, if you give the user the option to export TeX (so that they can then submit it to a journal).
(If you support both LaTeX and Typst, and improve the Overleaf experience somewhat -- which is definitely possible -- I can't imagine you wouldn't steal some market share from Overleaf.)
The Typst webapp [1] seems like it is already pretty similar to Overleaf.
LaTeX output seems not to be on the roadmap [2], which I can respect.
The amount of weirdness you would have to workaround to get good Typst to LaTeX translation sounds like a pain. I guess the hope is that the publishing industry starts adopting Typst...
