If you optimize a framework for beginners, you're optimizing for learnability. That's great for the first few days or even weeks of learning. But once you're past that, it's not so great. What you care more about is the usability of the thing once you know it. There are plenty of cases where learnability and usability are in conflict. Letting learnability win is a short-term relief.
If you on the other hand optimize for usability, for making things simple and easy to use for someone who understands the basics, you'll often end up with something that has great learnability as well. Maybe not as much as could be achieved as if that was your only goal, but plenty still.
The other aspect is whether you as a framework designer are designing something for yourself and your peers or if you're trying to dumb things down on behalf of beginners. In other words, designing for what you think beginners might find easy in their learning phase. That's really hard to do. Most technology I've seen designed in such a master/apprentice fashion sucks.
Finally, when you're comparing complexity, it's generally more helpful to look at the complexity involved with completing an entire task. Comparing complexity of "hello world" tells you little about what the complexity is going to be like developing a complete, modern web application from database migrations to XSS/CRSF protection.
That's where learnability and usability are again in conflict. Yes, it's harder to learn a framework that has answers for things like "how do I migrate a database once I have data I care about", or whatever feature you want to focus on, but once you hit that problem, you'll be glad that it's there and find the solution less complex than stringing something together from tons of separate parts yourself.
All that being said, I think Sinatra is wonderful. So what if it's not a fit for building a whole Basecamp or Github? It's great for teaching someone the basics of how HTTP works and it's great for smaller surface apps or for smaller, isolated parts of bigger apps.