Inventing a new language is cool and interesting. However, to invent a good language you need a considerable prior experience. After having invented a few languages you need to have used them extensively, make sober observations on what has gone right and what has gone wrong, and have the ability of figuring out how to fix the defects in the future without introducing new problems.
With templating languages, there is a problem. Not only are they being invented by authors doodling for the first time, and then we are getting stuck with all the unforeseen problems forever, but also the required features are not timely anticipated and implemented. In the end, patch by patch, hack after hack, we get a complete programming language, in a very ugly form.
In browsers, we already have a programming language. Some may not like it, but I argue that even if you take it for its worst, it is levels and levels above in its quality over the ugly templating abominations. Consider for a moment: no one forces an experienced programmer to write ugly and hacky code in JavaScript, while in templating languages, to achieve a certain goal, you are often mandated to write something monstrous.
Not only do we already have a programming language, but the one that has been around for quite a bit of time, and has seen much love. It manifested not only in ones among the fastest and most ingenious JIT compilers, but also recorded experience of rights and wrongs, good parts to use and bad parts to avoid, textbooks, articles, instruction material of all levels and grades of quality, where the best, hopefully, still can be found, and prevail.
So, the first upside is that you don't have to learn any new syntax and gain experience of using a new language. All the techniques you have learned are immediately applicable with frameorc. The second advantage is that all the tooling you have at your disposal still works: syntax colouring, editors, linters, formatters, debuggers, code analysis and generation tools and so on. Third, and no less important: all the syntax constructs of the language, and, if needed, libraries, work with your "markup" or "templates", while you are realising that your "templates" themselves can be organised in the usual way, by using functions and variables.
That means getting immense power without doing much other than understanding how things really are.