"Rails monoliths are so much more difficult to maintain"
I question that assertion. I have plenty of anecdotes which are the exact opposite: a Rails monolith is much easier to maintain compared to similar approaches in other languages.
"manipulate things globally"
Not sure what you mean by that to be quite honest. If you mean monkey-patching, that's generally been frowned upon except at the framework-level in the Ruby community for quite a while now.