Uhh, what? It being written as a single-page React Javascript app means that if you're a web dev with React under your belt, then it won't be tough for you to jump in and extend the CMS code[1]. And it's licensed under MIT, so you can fork and do what you will with it.

And you don't have to use Netlify's service with their Netlify-CMS. Their step-by-step guide uses it, but that's only because they obviously want to promote their service[2] and it's the easiest way to get it all set up. You'd otherwise just host the CMS page at /admin of your site and configure a webhook to deploy your site on a push event.

[1] Unless your point is just that programming is hard. Which is obviously true.

[2] Though their service is pretty shweet as they put your assets on cloudflare and handle the webhooks for you.