Agreed that MJML is great, but I wouldn't call JSX a useless abstraction. Using JSX as a templating language for MJML has real benefits, like being able to use javascript directly in templates instead of having to remember handlebars/mustache/nunjucks/etc templating syntax.
And libraries like mjml-react make it really easy.
And libraries like mjml-react make it really easy.
https://github.com/wix-incubator/mjml-react/