The REST API can be used for straight-up retrieval on any blog that has enabled it. Self-hosteds require an active choice to turn the JSON interface on in Jetpack but once it’s up it’s wide open.
This is not necessarily appreciated by everyone, and that’s somewhat understandable. I picked a blog ID at random here for the /posts/ call:
https://public-api.wordpress.com/rest/v1/sites/2833/posts/
That’s the full output. If you were trying to make life marginally harder for scrapers by only providing excerpts in your RSS feed then this pretty much undoes that. It’s a pretty false sense of security – if your blog is public then it’s scrapeable – but not everyone loves this fact.
For your purposes you can absolutely just use the API to pull individual posts and display them in an alternate manner.