why should they? this has been a worry for some time, but I don't see it happening

There's an active proposal for rendering html in canvas to control styling. Text rendering and accessibility are main focuses for the proposal.

https://github.com/WICG/html-in-canvas

I thought it was being done for web apps like Google Docs now, and I thought one of Microsoft's cross-platform toolkits used <canvas> too (though I don't think this toolkit itself will see broad adoption).

It already happened. WASM. And it's generally been used well.