> Except here the encapsulation comes at the cost of extending a core library at runtime

No, it doesn't.

Pervasive OO style and monkey-patching core classes are orthogonal. Ruby favors the former strongly, and supports the latter, and Rails rather heavily leverages the latter (to the extent that it has been harshly criticized for it by much of the non-Rails Ruby community), but there is no necessary relationship between them.