I call shenanigans.
It is perfect only if you hand-wave away the math and consequences.
You've doubled your surface area, number of apps, stacks, dependency issues, deployment targets, bugs, uptime dependencies, hiring language/framework requirements and attack vectors.
It is the right tool for some jobs. I'd encourage you to stop assuming any of this is done. People thought that when XML and Soap were the thing.
Also, it is completely unclear to me what 'old bullshit' is in your story. Is an API backend with UI front-end old or new? It could be read as that or as "API backend with JS front-end is the best, stop trying to push server-side rendered HTML on us."
This looks and feels a whole lot like what the Rails community was doing a decade ago with interpolated server-side values into ERB templates that contained script tags of jQuery. It seems like an old pattern that fell out of favor for good reasons (lack of fine grained control over the final rendered page's design and performance, still needing competencies in the front end tools when things don't work ideally, update and migration path difficulties for the front end, performance and caching issues, etc). I would personally not use Matestack as it seems like it has all the same pitfalls.