Aside from the usual separation of tech stacks for different teams, the big thing for me is lack of any sort of type hinting or safety in templates at least in the big frameworks such as Django, Rails etc. I would much rather work with a separate build process that utilizes typescript than deal with the errors that come out of incorrectly reading formless data and making typos within templates.
Is that really such a big problem? These days you can type annotate what you pass to the rendering function for templates and then you know what type you have in the template. If you have a minimum of testing, heck even manual testing will do, I don't think too many mistakes make it to staging, let alone production. I would think it well worth to be able to opt out of the JS ecosystem.