That's a problem for mathml. The more required tooling, the less likely it is to be used. TeX hits this weird sweet spot between writing code and writing a document that's very productive.
<p>
The triangle inequality states that
<la-tex>z \leq x + y</la-tex>.
</p>
I don’t think it is unreasonable to ask authors that insist on writing in familiar syntax, that they use a tool that enables that. With native MathML authors are free to use whichever tool chain they prefer. Some authors will prefer graphical editing of equations, so they have their authoring tools set up that way. Others will just want to write their familiar latex syntax so they’ll put a:
<script src="./path/to/texzilla.js"></script>