I shy away from answering questions like, is A going to replace B as I don't see it either being particularly constructive to A or B. I prefer to ask questions like what is the future of education? or how are MOOCs going to change our perceptions of education? 

As time becomes the most scarce commodity in our society, I see us valuing systems like the MOOC more and more. There will always be broken bits in every educational structure as every individual is different and it's almost impossible to create the perfect system that matches each distinctive learning style, be it in a formal structure or a relatively flexible one such as a MOOC. 

Ignatia deWaard in her book MOOC Yourself calls MOOCs a "non-defined pedagogical format to organise learning/teaching/training..." In this sense, perhaps even asking a question by juxtaposing MOOCs with formal education appears implausible as the structure of a MOOC is non-defined whilst attempting to organise learning nonetheless. By proposing a collaborative structure of learning, one not bound by hierarchical terms, learning is not singular but achieved in multiplicities. 

My curiosity into MOOCs arose from their differences to online courses or e-learning in its popular sense which is still one-directional and has clear objectives from the start.

MOOC work in a much more transient yet consistently pushing one's boundaries. Online courses could perhaps have been viewed as a threat against formal education as they functioned much like schools and universities did from an organisational perspective, whereas MOOCs operate from a primary objective to learn beyond any structural limitations. You learn so much in a MOOC whether you are a Mentor, Tutor, Participant or simply a Supporter and that's what sets it apart from formal education.

Do I think MOOCs can replace formal education? I hope so, but that's just because there is so much more potential in a MOOC that entirely rests upon every individual's choice and willingness to learn, however, the question is unanswerable - MOOCs by no means challenge formal education, although they certainly challenge institutional learning, especially how we internalise notions of learning & teaching as modes that cannot co-exist or as two aspects that should remain separate at all times.